55

app/app.py

@@ -19,7 +19,7 @@ def prepare_data():
     st.title("Sortie de la commande Python")
 
     # Commande Python à exécuter
-    command = "python3 ../data/shakespeare_char/prepare.py"
+    command = "python3 data/shakespeare_char/prepare.py"
 
     # Sorties de la commmande
     stdout, stderr = run_command(command)
@@ -43,7 +43,7 @@ def train_model():
     st.title("Sortie de la commande Python")
 
     # Commande Python à exécuter
-    command = "python3 ../train.py ../config/train_shakespeare_char.py --device=cpu --compile=False --eval_iters=20 --log_interval=1 --block_size=64 --batch_size=12 --n_layer=4 --n_head=4 --n_embd=128 --max_iters=2000 --lr_decay_iters=2000 --dropout=0.0"
+    command = "python3 train.py config/train_shakespeare_char.py --device=cpu --compile=False --eval_iters=20 --log_interval=1 --block_size=64 --batch_size=12 --n_layer=4 --n_head=4 --n_embd=128 --max_iters=2000 --lr_decay_iters=2000 --dropout=0.0"
 
     # Sorties de la commande
     stdout, stderr = run_command(command)

app/config/eval_gpt2.py

@@ -0,0 +1,8 @@
+# evaluate the base gpt2
+# n_layer=12, n_head=12, n_embd=768
+# 124M parameters
+batch_size = 8
+eval_iters = 500 # use more iterations to get good estimate
+eval_only = True
+wandb_log = False
+init_from = 'gpt2'

app/config/eval_gpt2_large.py

@@ -0,0 +1,8 @@
+# evaluate the base gpt2
+# n_layer=36, n_head=20, n_embd=1280
+# 774M parameters
+batch_size = 8
+eval_iters = 500 # use more iterations to get good estimate
+eval_only = True
+wandb_log = False
+init_from = 'gpt2-large'

app/config/eval_gpt2_medium.py

@@ -0,0 +1,8 @@
+# evaluate the base gpt2
+# n_layer=24, n_head=16, n_embd=1024
+# 350M parameters
+batch_size = 8
+eval_iters = 500 # use more iterations to get good estimate
+eval_only = True
+wandb_log = False
+init_from = 'gpt2-medium'

app/config/eval_gpt2_xl.py

@@ -0,0 +1,8 @@
+# evaluate the base gpt2
+# n_layer=48, n_head=25, n_embd=1600
+# 1558M parameters
+batch_size = 8
+eval_iters = 500 # use more iterations to get good estimate
+eval_only = True
+wandb_log = False
+init_from = 'gpt2-xl'

app/config/finetune_shakespeare.py

@@ -0,0 +1,25 @@
+import time
+
+out_dir = 'out-shakespeare'
+eval_interval = 5
+eval_iters = 40
+wandb_log = False # feel free to turn on
+wandb_project = 'shakespeare'
+wandb_run_name = 'ft-' + str(time.time())
+
+dataset = 'shakespeare'
+init_from = 'gpt2-xl' # this is the largest GPT-2 model
+
+# only save checkpoints if the validation loss improves
+always_save_checkpoint = False
+
+# the number of examples per iter:
+# 1 batch_size * 32 grad_accum * 1024 tokens = 32,768 tokens/iter
+# shakespeare has 301,966 tokens, so 1 epoch ~= 9.2 iters
+batch_size = 1
+gradient_accumulation_steps = 32
+max_iters = 20
+
+# finetune at constant LR
+learning_rate = 3e-5
+decay_lr = False

app/config/train_gpt2.py

@@ -0,0 +1,25 @@
+# config for training GPT-2 (124M) down to very nice loss of ~2.85 on 1 node of 8X A100 40GB
+# launch as the following (e.g. in a screen session) and wait ~5 days:
+# $ torchrun --standalone --nproc_per_node=8 train.py config/train_gpt2.py
+
+wandb_log = True
+wandb_project = 'owt'
+wandb_run_name='gpt2-124M'
+
+# these make the total batch size be ~0.5M
+# 12 batch size * 1024 block size * 5 gradaccum * 8 GPUs = 491,520
+batch_size = 12
+block_size = 1024
+gradient_accumulation_steps = 5 * 8
+
+# this makes total number of tokens be 300B
+max_iters = 600000
+lr_decay_iters = 600000
+
+# eval stuff
+eval_interval = 1000
+eval_iters = 200
+log_interval = 10
+
+# weight decay
+weight_decay = 1e-1

app/config/train_shakespeare_char.py

@@ -0,0 +1,37 @@
+# train a miniature character-level shakespeare model
+# good for debugging and playing on macbooks and such
+
+out_dir = 'out-shakespeare-char'
+eval_interval = 250 # keep frequent because we'll overfit
+eval_iters = 200
+log_interval = 10 # don't print too too often
+
+# we expect to overfit on this small dataset, so only save when val improves
+always_save_checkpoint = False
+
+wandb_log = False # override via command line if you like
+wandb_project = 'shakespeare-char'
+wandb_run_name = 'mini-gpt'
+
+dataset = 'shakespeare_char'
+gradient_accumulation_steps = 1
+batch_size = 64
+block_size = 256 # context of up to 256 previous characters
+
+# baby GPT model :)
+n_layer = 6
+n_head = 6
+n_embd = 384
+dropout = 0.2
+
+learning_rate = 1e-3 # with baby networks can afford to go a bit higher
+max_iters = 5000
+lr_decay_iters = 5000 # make equal to max_iters usually
+min_lr = 1e-4 # learning_rate / 10 usually
+beta2 = 0.99 # make a bit bigger because number of tokens per iter is small
+
+warmup_iters = 100 # not super necessary potentially
+
+# on macbook also add
+# device = 'cpu'  # run on cpu only
+# compile = False # do not torch compile the model

app/data/openwebtext/prepare.py

@@ -0,0 +1,80 @@
+# saves the openwebtext dataset to a binary file for training. following was helpful:
+# https://github.com/HazyResearch/flash-attention/blob/main/training/src/datamodules/language_modeling_hf.py
+
+import os
+from tqdm import tqdm
+import numpy as np
+import tiktoken
+from datasets import load_dataset # huggingface datasets
+
+# number of workers in .map() call
+# good number to use is ~order number of cpu cores // 2
+num_proc = 8
+
+# number of workers in load_dataset() call
+# best number might be different from num_proc above as it also depends on NW speed.
+# it is better than 1 usually though
+num_proc_load_dataset = num_proc
+
+if __name__ == '__main__':
+    # takes 54GB in huggingface .cache dir, about 8M documents (8,013,769)
+    dataset = load_dataset("openwebtext", num_proc=num_proc_load_dataset)
+
+    # owt by default only contains the 'train' split, so create a test split
+    split_dataset = dataset["train"].train_test_split(test_size=0.0005, seed=2357, shuffle=True)
+    split_dataset['val'] = split_dataset.pop('test') # rename the test split to val
+
+    # this results in:
+    # >>> split_dataset
+    # DatasetDict({
+    #     train: Dataset({
+    #         features: ['text'],
+    #         num_rows: 8009762
+    #     })
+    #     val: Dataset({
+    #         features: ['text'],
+    #         num_rows: 4007
+    #     })
+    # })
+
+    # we now want to tokenize the dataset. first define the encoding function (gpt2 bpe)
+    enc = tiktoken.get_encoding("gpt2")
+    def process(example):
+        ids = enc.encode_ordinary(example['text']) # encode_ordinary ignores any special tokens
+        ids.append(enc.eot_token) # add the end of text token, e.g. 50256 for gpt2 bpe
+        # note: I think eot should be prepended not appended... hmm. it's called "eot" though...
+        out = {'ids': ids, 'len': len(ids)}
+        return out
+
+    # tokenize the dataset
+    tokenized = split_dataset.map(
+        process,
+        remove_columns=['text'],
+        desc="tokenizing the splits",
+        num_proc=num_proc,
+    )
+
+    # concatenate all the ids in each dataset into one large file we can use for training
+    for split, dset in tokenized.items():
+        arr_len = np.sum(dset['len'], dtype=np.uint64)
+        filename = os.path.join(os.path.dirname(__file__), f'{split}.bin')
+        dtype = np.uint16 # (can do since enc.max_token_value == 50256 is < 2**16)
+        arr = np.memmap(filename, dtype=dtype, mode='w+', shape=(arr_len,))
+        total_batches = 1024
+
+        idx = 0
+        for batch_idx in tqdm(range(total_batches), desc=f'writing {filename}'):
+            # Batch together samples for faster write
+            batch = dset.shard(num_shards=total_batches, index=batch_idx, contiguous=True).with_format('numpy')
+            arr_batch = np.concatenate(batch['ids'])
+            # Write into mmap
+            arr[idx : idx + len(arr_batch)] = arr_batch
+            idx += len(arr_batch)
+        arr.flush()
+
+    # train.bin is ~17GB, val.bin ~8.5MB
+    # train has ~9B tokens (9,035,582,198)
+    # val has ~4M tokens (4,434,897)
+
+    # to read the bin files later, e.g. with numpy:
+    # m = np.memmap('train.bin', dtype=np.uint16, mode='r')

app/data/openwebtext/readme.md

@@ -0,0 +1,15 @@
+
+## openwebtext dataset
+
+after running `prepare.py` (preprocess) we get:
+
+- train.bin is ~17GB, val.bin ~8.5MB
+- train has ~9B tokens (9,035,582,198)
+- val has ~4M tokens (4,434,897)
+
+this came from 8,013,769 documents in total.
+
+references:
+
+- OpenAI's WebText dataset is discussed in [GPT-2 paper](https://d4mucfpksywv.cloudfront.net/better-language-models/language_models_are_unsupervised_multitask_learners.pdf)
+- [OpenWebText](https://skylion007.github.io/OpenWebTextCorpus/) dataset

app/data/shakespeare/prepare.py

@@ -0,0 +1,33 @@
+import os
+import requests
+import tiktoken
+import numpy as np
+
+# download the tiny shakespeare dataset
+input_file_path = os.path.join(os.path.dirname(__file__), 'input.txt')
+if not os.path.exists(input_file_path):
+    data_url = 'https://raw.githubusercontent.com/karpathy/char-rnn/master/data/tinyshakespeare/input.txt'
+    with open(input_file_path, 'w') as f:
+        f.write(requests.get(data_url).text)
+
+with open(input_file_path, 'r') as f:
+    data = f.read()
+n = len(data)
+train_data = data[:int(n*0.9)]
+val_data = data[int(n*0.9):]
+
+# encode with tiktoken gpt2 bpe
+enc = tiktoken.get_encoding("gpt2")
+train_ids = enc.encode_ordinary(train_data)
+val_ids = enc.encode_ordinary(val_data)
+print(f"train has {len(train_ids):,} tokens")
+print(f"val has {len(val_ids):,} tokens")
+
+# export to bin files
+train_ids = np.array(train_ids, dtype=np.uint16)
+val_ids = np.array(val_ids, dtype=np.uint16)
+train_ids.tofile(os.path.join(os.path.dirname(__file__), 'train.bin'))
+val_ids.tofile(os.path.join(os.path.dirname(__file__), 'val.bin'))
+
+# train.bin has 301,966 tokens
+# val.bin has 36,059 tokens

app/data/shakespeare/readme.md

@@ -0,0 +1,9 @@
+
+# tiny shakespeare
+
+Tiny shakespeare, of the good old char-rnn fame :)
+
+After running `prepare.py`:
+
+- train.bin has 301,966 tokens
+- val.bin has 36,059 tokens

app/data/shakespeare_char/prepare.py

@@ -0,0 +1,68 @@
+"""
+Prepare the Shakespeare dataset for character-level language modeling.
+So instead of encoding with GPT-2 BPE tokens, we just map characters to ints.
+Will save train.bin, val.bin containing the ids, and meta.pkl containing the
+encoder and decoder and some other related info.
+"""
+import os
+import pickle
+import requests
+import numpy as np
+
+# download the tiny shakespeare dataset
+input_file_path = os.path.join(os.path.dirname(__file__), 'input.txt')
+if not os.path.exists(input_file_path):
+    data_url = 'https://raw.githubusercontent.com/karpathy/char-rnn/master/data/tinyshakespeare/input.txt'
+    with open(input_file_path, 'w') as f:
+        f.write(requests.get(data_url).text)
+
+with open(input_file_path, 'r') as f:
+    data = f.read()
+print(f"length of dataset in characters: {len(data):,}")
+
+# get all the unique characters that occur in this text
+chars = sorted(list(set(data)))
+vocab_size = len(chars)
+print("all the unique characters:", ''.join(chars))
+print(f"vocab size: {vocab_size:,}")
+
+# create a mapping from characters to integers
+stoi = { ch:i for i,ch in enumerate(chars) }
+itos = { i:ch for i,ch in enumerate(chars) }
+def encode(s):
+    return [stoi[c] for c in s] # encoder: take a string, output a list of integers
+def decode(l):
+    return ''.join([itos[i] for i in l]) # decoder: take a list of integers, output a string
+
+# create the train and test splits
+n = len(data)
+train_data = data[:int(n*0.9)]
+val_data = data[int(n*0.9):]
+
+# encode both to integers
+train_ids = encode(train_data)
+val_ids = encode(val_data)
+print(f"train has {len(train_ids):,} tokens")
+print(f"val has {len(val_ids):,} tokens")
+
+# export to bin files
+train_ids = np.array(train_ids, dtype=np.uint16)
+val_ids = np.array(val_ids, dtype=np.uint16)
+train_ids.tofile(os.path.join(os.path.dirname(__file__), 'train.bin'))
+val_ids.tofile(os.path.join(os.path.dirname(__file__), 'val.bin'))
+
+# save the meta information as well, to help us encode/decode later
+meta = {
+    'vocab_size': vocab_size,
+    'itos': itos,
+    'stoi': stoi,
+}
+with open(os.path.join(os.path.dirname(__file__), 'meta.pkl'), 'wb') as f:
+    pickle.dump(meta, f)
+
+# length of dataset in characters:  1115394
+# all the unique characters:
+#  !$&',-.3:;?ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz
+# vocab size: 65
+# train has 1003854 tokens
+# val has 111540 tokens

app/data/shakespeare_char/readme.md

@@ -0,0 +1,9 @@
+
+# tiny shakespeare, character-level
+
+Tiny shakespeare, of the good old char-rnn fame :) Treated on character-level.
+
+After running `prepare.py`:
+
+- train.bin has 1,003,854 tokens
+- val.bin has 111,540 tokens

app/train.py

@@ -0,0 +1,333 @@
+"""
+This training script can be run both on a single gpu in debug mode,
+and also in a larger training run with distributed data parallel (ddp).
+
+To run on a single GPU, example:
+$ python train.py --batch_size=32 --compile=False
+
+To run with DDP on 4 gpus on 1 node, example:
+$ torchrun --standalone --nproc_per_node=4 train.py
+
+To run with DDP on 4 gpus across 2 nodes, example:
+- Run on the first (master) node with example IP 123.456.123.456:
+$ torchrun --nproc_per_node=8 --nnodes=2 --node_rank=0 --master_addr=123.456.123.456 --master_port=1234 train.py
+- Run on the worker node:
+$ torchrun --nproc_per_node=8 --nnodes=2 --node_rank=1 --master_addr=123.456.123.456 --master_port=1234 train.py
+(If your cluster does not have Infiniband interconnect prepend NCCL_IB_DISABLE=1)
+"""
+
+import os
+import time
+import math
+import pickle
+from contextlib import nullcontext
+
+import numpy as np
+import torch
+from torch.nn.parallel import DistributedDataParallel as DDP
+from torch.distributed import init_process_group, destroy_process_group
+
+from model import GPTConfig, GPT
+
+# -----------------------------------------------------------------------------
+# default config values designed to train a gpt2 (124M) on OpenWebText
+# I/O
+out_dir = 'out'
+eval_interval = 2000
+log_interval = 1
+eval_iters = 200
+eval_only = False # if True, script exits right after the first eval
+always_save_checkpoint = True # if True, always save a checkpoint after each eval
+init_from = 'scratch' # 'scratch' or 'resume' or 'gpt2*'
+# wandb logging
+wandb_log = False # disabled by default
+wandb_project = 'owt'
+wandb_run_name = 'gpt2' # 'run' + str(time.time())
+# data
+dataset = 'openwebtext'
+gradient_accumulation_steps = 5 * 8 # used to simulate larger batch sizes
+batch_size = 12 # if gradient_accumulation_steps > 1, this is the micro-batch size
+block_size = 1024
+# model
+n_layer = 12
+n_head = 12
+n_embd = 768
+dropout = 0.0 # for pretraining 0 is good, for finetuning try 0.1+
+bias = False # do we use bias inside LayerNorm and Linear layers?
+# adamw optimizer
+learning_rate = 6e-4 # max learning rate
+max_iters = 600000 # total number of training iterations
+weight_decay = 1e-1
+beta1 = 0.9
+beta2 = 0.95
+grad_clip = 1.0 # clip gradients at this value, or disable if == 0.0
+# learning rate decay settings
+decay_lr = True # whether to decay the learning rate
+warmup_iters = 2000 # how many steps to warm up for
+lr_decay_iters = 600000 # should be ~= max_iters per Chinchilla
+min_lr = 6e-5 # minimum learning rate, should be ~= learning_rate/10 per Chinchilla
+# DDP settings
+backend = 'nccl' # 'nccl', 'gloo', etc.
+# system
+device = 'cuda' # examples: 'cpu', 'cuda', 'cuda:0', 'cuda:1' etc., or try 'mps' on macbooks
+dtype = 'bfloat16' if torch.cuda.is_available() and torch.cuda.is_bf16_supported() else 'float16' # 'float32', 'bfloat16', or 'float16', the latter will auto implement a GradScaler
+compile = True # use PyTorch 2.0 to compile the model to be faster
+# -----------------------------------------------------------------------------
+config_keys = [k for k,v in globals().items() if not k.startswith('_') and isinstance(v, (int, float, bool, str))]
+exec(open('configurator.py').read()) # overrides from command line or config file
+config = {k: globals()[k] for k in config_keys} # will be useful for logging
+# -----------------------------------------------------------------------------
+
+# various inits, derived attributes, I/O setup
+ddp = int(os.environ.get('RANK', -1)) != -1 # is this a ddp run?
+if ddp:
+    init_process_group(backend=backend)
+    ddp_rank = int(os.environ['RANK'])
+    ddp_local_rank = int(os.environ['LOCAL_RANK'])
+    ddp_world_size = int(os.environ['WORLD_SIZE'])
+    device = f'cuda:{ddp_local_rank}'
+    torch.cuda.set_device(device)
+    master_process = ddp_rank == 0 # this process will do logging, checkpointing etc.
+    seed_offset = ddp_rank # each process gets a different seed
+    # world_size number of processes will be training simultaneously, so we can scale
+    # down the desired gradient accumulation iterations per process proportionally
+    assert gradient_accumulation_steps % ddp_world_size == 0
+    gradient_accumulation_steps //= ddp_world_size
+else:
+    # if not ddp, we are running on a single gpu, and one process
+    master_process = True
+    seed_offset = 0
+    ddp_world_size = 1
+tokens_per_iter = gradient_accumulation_steps * ddp_world_size * batch_size * block_size
+print(f"tokens per iteration will be: {tokens_per_iter:,}")
+
+if master_process:
+    os.makedirs(out_dir, exist_ok=True)
+torch.manual_seed(1337 + seed_offset)
+torch.backends.cuda.matmul.allow_tf32 = True # allow tf32 on matmul
+torch.backends.cudnn.allow_tf32 = True # allow tf32 on cudnn
+device_type = 'cuda' if 'cuda' in device else 'cpu' # for later use in torch.autocast
+# note: float16 data type will automatically use a GradScaler
+ptdtype = {'float32': torch.float32, 'bfloat16': torch.bfloat16, 'float16': torch.float16}[dtype]
+ctx = nullcontext() if device_type == 'cpu' else torch.amp.autocast(device_type=device_type, dtype=ptdtype)
+
+# poor man's data loader
+data_dir = os.path.join('data', dataset)
+train_data = np.memmap(os.path.join(data_dir, 'train.bin'), dtype=np.uint16, mode='r')
+val_data = np.memmap(os.path.join(data_dir, 'val.bin'), dtype=np.uint16, mode='r')
+def get_batch(split):
+    data = train_data if split == 'train' else val_data
+    ix = torch.randint(len(data) - block_size, (batch_size,))
+    x = torch.stack([torch.from_numpy((data[i:i+block_size]).astype(np.int64)) for i in ix])
+    y = torch.stack([torch.from_numpy((data[i+1:i+1+block_size]).astype(np.int64)) for i in ix])
+    if device_type == 'cuda':
+        # pin arrays x,y, which allows us to move them to GPU asynchronously (non_blocking=True)
+        x, y = x.pin_memory().to(device, non_blocking=True), y.pin_memory().to(device, non_blocking=True)
+    else:
+        x, y = x.to(device), y.to(device)
+    return x, y
+
+# init these up here, can override if init_from='resume' (i.e. from a checkpoint)
+iter_num = 0
+best_val_loss = 1e9
+
+# attempt to derive vocab_size from the dataset
+meta_path = os.path.join(data_dir, 'meta.pkl')
+meta_vocab_size = None
+if os.path.exists(meta_path):
+    with open(meta_path, 'rb') as f:
+        meta = pickle.load(f)
+    meta_vocab_size = meta['vocab_size']
+    print(f"found vocab_size = {meta_vocab_size} (inside {meta_path})")
+
+# model init
+model_args = dict(n_layer=n_layer, n_head=n_head, n_embd=n_embd, block_size=block_size,
+                  bias=bias, vocab_size=None, dropout=dropout) # start with model_args from command line
+if init_from == 'scratch':
+    # init a new model from scratch
+    print("Initializing a new model from scratch")
+    # determine the vocab size we'll use for from-scratch training
+    if meta_vocab_size is None:
+        print("defaulting to vocab_size of GPT-2 to 50304 (50257 rounded up for efficiency)")
+    model_args['vocab_size'] = meta_vocab_size if meta_vocab_size is not None else 50304
+    gptconf = GPTConfig(**model_args)
+    model = GPT(gptconf)
+elif init_from == 'resume':
+    print(f"Resuming training from {out_dir}")
+    # resume training from a checkpoint.
+    ckpt_path = os.path.join(out_dir, 'ckpt.pt')
+    checkpoint = torch.load(ckpt_path, map_location=device)
+    checkpoint_model_args = checkpoint['model_args']
+    # force these config attributes to be equal otherwise we can't even resume training
+    # the rest of the attributes (e.g. dropout) can stay as desired from command line
+    for k in ['n_layer', 'n_head', 'n_embd', 'block_size', 'bias', 'vocab_size']:
+        model_args[k] = checkpoint_model_args[k]
+    # create the model
+    gptconf = GPTConfig(**model_args)
+    model = GPT(gptconf)
+    state_dict = checkpoint['model']
+    # fix the keys of the state dictionary :(
+    # honestly no idea how checkpoints sometimes get this prefix, have to debug more
+    unwanted_prefix = '_orig_mod.'
+    for k,v in list(state_dict.items()):
+        if k.startswith(unwanted_prefix):
+            state_dict[k[len(unwanted_prefix):]] = state_dict.pop(k)
+    model.load_state_dict(state_dict)
+    iter_num = checkpoint['iter_num']
+    best_val_loss = checkpoint['best_val_loss']
+elif init_from.startswith('gpt2'):
+    print(f"Initializing from OpenAI GPT-2 weights: {init_from}")
+    # initialize from OpenAI GPT-2 weights
+    override_args = dict(dropout=dropout)
+    model = GPT.from_pretrained(init_from, override_args)
+    # read off the created config params, so we can store them into checkpoint correctly
+    for k in ['n_layer', 'n_head', 'n_embd', 'block_size', 'bias', 'vocab_size']:
+        model_args[k] = getattr(model.config, k)
+# crop down the model block size if desired, using model surgery
+if block_size < model.config.block_size:
+    model.crop_block_size(block_size)
+    model_args['block_size'] = block_size # so that the checkpoint will have the right value
+model.to(device)
+
+# initialize a GradScaler. If enabled=False scaler is a no-op
+scaler = torch.cuda.amp.GradScaler(enabled=(dtype == 'float16'))
+
+# optimizer
+optimizer = model.configure_optimizers(weight_decay, learning_rate, (beta1, beta2), device_type)
+if init_from == 'resume':
+    optimizer.load_state_dict(checkpoint['optimizer'])
+checkpoint = None # free up memory
+
+# compile the model
+if compile:
+    print("compiling the model... (takes a ~minute)")
+    unoptimized_model = model
+    model = torch.compile(model) # requires PyTorch 2.0
+
+# wrap model into DDP container
+if ddp:
+    model = DDP(model, device_ids=[ddp_local_rank])
+
+# helps estimate an arbitrarily accurate loss over either split using many batches
+@torch.no_grad()
+def estimate_loss():
+    out = {}
+    model.eval()
+    for split in ['train', 'val']:
+        losses = torch.zeros(eval_iters)
+        for k in range(eval_iters):
+            X, Y = get_batch(split)
+            with ctx:
+                logits, loss = model(X, Y)
+            losses[k] = loss.item()
+        out[split] = losses.mean()
+    model.train()
+    return out
+
+# learning rate decay scheduler (cosine with warmup)
+def get_lr(it):
+    # 1) linear warmup for warmup_iters steps
+    if it < warmup_iters:
+        return learning_rate * it / warmup_iters
+    # 2) if it > lr_decay_iters, return min learning rate
+    if it > lr_decay_iters:
+        return min_lr
+    # 3) in between, use cosine decay down to min learning rate
+    decay_ratio = (it - warmup_iters) / (lr_decay_iters - warmup_iters)
+    assert 0 <= decay_ratio <= 1
+    coeff = 0.5 * (1.0 + math.cos(math.pi * decay_ratio)) # coeff ranges 0..1
+    return min_lr + coeff * (learning_rate - min_lr)
+
+# logging
+if wandb_log and master_process:
+    import wandb
+    wandb.init(project=wandb_project, name=wandb_run_name, config=config)
+
+# training loop
+X, Y = get_batch('train') # fetch the very first batch
+t0 = time.time()
+local_iter_num = 0 # number of iterations in the lifetime of this process
+raw_model = model.module if ddp else model # unwrap DDP container if needed
+running_mfu = -1.0
+while True:
+
+    # determine and set the learning rate for this iteration
+    lr = get_lr(iter_num) if decay_lr else learning_rate
+    for param_group in optimizer.param_groups:
+        param_group['lr'] = lr
+
+    # evaluate the loss on train/val sets and write checkpoints
+    if iter_num % eval_interval == 0 and master_process:
+        losses = estimate_loss()
+        print(f"step {iter_num}: train loss {losses['train']:.4f}, val loss {losses['val']:.4f}")
+        if wandb_log:
+            wandb.log({
+                "iter": iter_num,
+                "train/loss": losses['train'],
+                "val/loss": losses['val'],
+                "lr": lr,
+                "mfu": running_mfu*100, # convert to percentage
+            })
+        if losses['val'] < best_val_loss or always_save_checkpoint:
+            best_val_loss = losses['val']
+            if iter_num > 0:
+                checkpoint = {
+                    'model': raw_model.state_dict(),
+                    'optimizer': optimizer.state_dict(),
+                    'model_args': model_args,
+                    'iter_num': iter_num,
+                    'best_val_loss': best_val_loss,
+                    'config': config,
+                }
+                print(f"saving checkpoint to {out_dir}")
+                torch.save(checkpoint, os.path.join(out_dir, 'ckpt.pt'))
+    if iter_num == 0 and eval_only:
+        break
+
+    # forward backward update, with optional gradient accumulation to simulate larger batch size
+    # and using the GradScaler if data type is float16
+    for micro_step in range(gradient_accumulation_steps):
+        if ddp:
+            # in DDP training we only need to sync gradients at the last micro step.
+            # the official way to do this is with model.no_sync() context manager, but
+            # I really dislike that this bloats the code and forces us to repeat code
+            # looking at the source of that context manager, it just toggles this variable
+            model.require_backward_grad_sync = (micro_step == gradient_accumulation_steps - 1)
+        with ctx:
+            logits, loss = model(X, Y)
+            loss = loss / gradient_accumulation_steps # scale the loss to account for gradient accumulation
+        # immediately async prefetch next batch while model is doing the forward pass on the GPU
+        X, Y = get_batch('train')
+        # backward pass, with gradient scaling if training in fp16
+        scaler.scale(loss).backward()
+    # clip the gradient
+    if grad_clip != 0.0:
+        scaler.unscale_(optimizer)
+        torch.nn.utils.clip_grad_norm_(model.parameters(), grad_clip)
+    # step the optimizer and scaler if training in fp16
+    scaler.step(optimizer)
+    scaler.update()
+    # flush the gradients as soon as we can, no need for this memory anymore
+    optimizer.zero_grad(set_to_none=True)
+
+    # timing and logging
+    t1 = time.time()
+    dt = t1 - t0
+    t0 = t1
+    if iter_num % log_interval == 0 and master_process:
+        # get loss as float. note: this is a CPU-GPU sync point
+        # scale up to undo the division above, approximating the true total loss (exact would have been a sum)
+        lossf = loss.item() * gradient_accumulation_steps
+        if local_iter_num >= 5: # let the training loop settle a bit
+            mfu = raw_model.estimate_mfu(batch_size * gradient_accumulation_steps, dt)
+            running_mfu = mfu if running_mfu == -1.0 else 0.9*running_mfu + 0.1*mfu
+        print(f"iter {iter_num}: loss {lossf:.4f}, time {dt*1000:.2f}ms, mfu {running_mfu*100:.2f}%")
+    iter_num += 1
+    local_iter_num += 1
+
+    # termination conditions
+    if iter_num > max_iters:
+        break
+
+if ddp:
+    destroy_process_group()