The batch_size in OPTModel limits the training performance with Pytorch FSDP
Author: young-chao
Pull request: no

### System Info

When I use transformers' OPTModel to load the opt-13b model for training with Pytorch FSDP, I found that the whole training is limited by batch_size. Although FSDP has the ability to offload parameters to the CPU memory to reduce the pressure on the GPU memory, due to the impact of batch on the parameter scale of the forward phase, the GPU memory overflows when some parameters are initialized on the GPU.

### Who can help?

@sgugger @ArthurZucker @younesbelkada

### Information

- [ ] The official example scripts
- [X] My own modified scripts

### Tasks

- [ ] An officially supported task in the `examples` folder (such as GLUE/SQuAD, ...)
- [X] My own task or dataset (give details below)

### Reproduction

 ### Training code
```python
import os
import argparse
import functools
import torch
from itertools import chain
import torch.nn as nn
import torch.optim as optim
from transformers import (
    OPTForCausalLM,
    AutoTokenizer,
    default_data_collator,
)
from transformers.models.opt.modeling_opt import OPTDecoderLayer, OPTAttention
from datasets import load_dataset
from torch.utils.data import DataLoader
from torch.optim.lr_scheduler import StepLR

import torch.distributed as dist
import torch.multiprocessing as mp
from torch.distributed.fsdp import (
    MixedPrecision,
    FullyShardedDataParallel as FSDP
)
from torch.distributed.fsdp.fully_sharded_data_parallel import (
    CPUOffload,
)
from torch.distributed.fsdp.wrap import (
    size_based_auto_wrap_policy,
    transformer_auto_wrap_policy,
)
from torch.distributed.algorithms._checkpoint.checkpoint_wrapper import (
    checkpoint_wrapper,
)


def getDataset():
    raw_datasets = load_dataset("wikitext", "wikitext-2-v1")
    tokenizer = AutoTokenizer.from_pretrained("facebook/opt-13b")
    column_names = raw_datasets["train"].column_names
    text_column_name = "text" if "text" in column_names else column_names[0]

    def tokenize_function(examples):
        return tokenizer(examples[text_column_name])

    tokenized_datasets = raw_datasets.map(
        tokenize_function,
        batched=True,
        num_proc=1,
        remove_columns=column_names,
        load_from_cache_file=False,
        desc="Running tokenizer on dataset",
    )

    def group_texts(examples):
        # Concatenate all texts.
        concatenated_examples = {
            k: list(chain(*examples[k])) for k in examples.keys()}
        total_length = len(concatenated_examples[list(examples.keys())[0]])
        # We drop the small remainder, we could add padding if the model supported it instead of this drop, you can
        # customize this part to your needs.
        if total_length >= 1024:
            total_length = (total_length // 1024) * 1024
        # Split by chunks of max_len.
        result = {
            k: [t[i: i + 1024]
                for i in range(0, total_length, 1024)]
            for k, t in concatenated_examples.items()
        }
        result["labels"] = result["input_ids"].copy()
        return result

    lm_datasets = tokenized_datasets.map(
        group_texts,
        batched=True,
        num_proc=1,
        load_from_cache_file=False,
        desc=f"Grouping texts in chunks of {1024}",
    )

    return lm_datasets["train"]


def setup(rank, world_size):
    os.environ['MASTER_ADDR'] = 'localhost'
    os.environ['MASTER_PORT'] = '12355'

    # initialize the process group
    dist.init_process_group("nccl", rank=rank, world_size=world_size)


def cleanup():
    dist.destroy_process_group()


def train(args, model, rank, world_size, train_loader, optimizer, epoch):
    model.train()
    ddp_loss = torch.zeros(2).to(rank)
    for batch_idx, batch in enumerate(train_loader):
        input_ids = batch["input_ids"].to(rank)
        attention_mask = batch["attention_mask"].to(rank)
        labels = batch["labels"].to(rank)
        print(rank, "start forward", batch_idx, " *"*10)
        outputs = model(input_ids=input_ids,
                        attention_mask=attention_mask, labels=labels)
        optimizer.zero_grad()
        loss = outputs.loss
        print(rank, "start backward", batch_idx, " *"*10)
        loss.backward()
        optimizer.step()
        ddp_loss[0] += loss.item()
        ddp_loss[1] += len(input_ids)
        if rank == 0:
            print(batch_idx, " *"*10)

    dist.all_reduce(ddp_loss, op=dist.ReduceOp.SUM)
    if rank == 0:
        print('Train Epoch: {} \tLoss: {:.6f}'.format(
            epoch, ddp_loss[0] / ddp_loss[1]))


def fsdp_main(rank, world_size, args):
    setup(rank, world_size)

    train_dataset = getDataset()
    train_loader = DataLoader(
        train_dataset, collate_fn=default_data_collator,
        batch_size=101, num_workers=1
    )

    my_auto_wrap_policy = functools.partial(
        size_based_auto_wrap_policy, min_num_params=100000
    )
    # my_auto_wrap_policy = functools.partial(
    #     transformer_auto_wrap_policy, transformer_layer_cls={
    #         OPTDecoderLayer, OPTAttention, nn.LayerNorm, nn.Linear}
    # )
    torch.cuda.set_device(rank)

    init_start_event = torch.cuda.Event(enable_timing=True)
    init_end_event = torch.cuda.Event(enable_timing=True)

    if rank == 0:
        print("*"*10+"loading to cpu"+"*"*10)
    model = OPTForCausalLM.from_pretrained("facebook/opt-13b")
    model = checkpoint_wrapper(model, offload_to_cpu=True)

    model = FSDP(model,
                 cpu_offload=CPUOffload(CPUOffload(offload_params=True)),
                 auto_wrap_policy=my_auto_wrap_policy,
                 mixed_precision=MixedPrecision(param_dtype=torch.float16,
                                                reduce_dtype=torch.float16,
                                                buffer_dtype=torch.float16,
                                                keep_low_precision_grads=True)
                 )
    if rank == 0:
        print("*"*10+"print the fsdp model"+"*"*10)
        print(model)
        print_file = open("./model", 'w')
        print(model, file=print_file)
        print()

    optimizer = optim.Adam(model.parameters(), lr=args.lr)
    # optimizer = optim.SGD(model.parameters(), lr=args.lr)

    scheduler = StepLR(optimizer, step_size=1, gamma=args.gamma)
    init_start_event.record()
    for epoch in range(1, args.epochs + 1):
        train(args, model, rank, world_size, train_loader,
              optimizer, epoch)
        scheduler.step()

    init_end_event.record()

    if rank == 0:
        print(
            f"CUDA event elapsed time: {init_start_event.elapsed_time(init_end_event) / 1000}sec")
        print(f"{model}")

    cleanup()


if __name__ == '__main__':
    # Training settings
    parser = argparse.ArgumentParser(description='PyTorch OPT Example')
    parser.add_argument('--batch-size', type=int, default=1, metavar='N',
                        help='input batch size for training (default: 64)')
    parser.add_argument('--epochs', type=int, default=1, metavar='N',
                        help='number of epochs to train (default: 14)')
    parser.add_argument('--lr', type=float, default=0.001, metavar='LR',
                        help='learning rate (default: 0.001)')
    parser.add_argument('--gamma', type=float, default=0.7, metavar='M',
                        help='Learning rate step gamma (default: 0.7)')
    parser.add_argument('--no-cuda', action='store_true', default=False,
                        help='disables CUDA training')
    parser.add_argument('--seed', type=int, default=1, metavar='S',
                        help='random seed (default: 1)')
    args = parser.parse_args()

    torch.manual_seed(args.seed)

    WORLD_SIZE = torch.cuda.device_count()
    mp.spawn(fsdp_main,
             args=(WORLD_SIZE, args),
             nprocs=WORLD_SIZE,
             join=True)
```

### Expected behavior

The shape of attn_weights is (bsz：100，self.num_heads：40，tgt_len：1024，src_len：1024). Even though its data type is fp16, its size has reached close to 8GB, which directly leads to gpu memory overflow.

![image](https://user-images.githubusercontent.com/34190033/218054752-78ab6831-82e8-4018-a0a0-93a655bfbb36.png)


Comments:
The bsz is actually the input batch_size.

Seems like the crash happens during the forward pass, and is not directly related to the transformers library.
This does not seem like a bug but rather a discussion, feel free to ask on [the forum](https://discuss.huggingface.co/).