Experimental support for fairscale ShardedDDP
# What does this PR do?

This PR adds support for [FairScale](https://github.com/facebookresearch/fairscale)'s shared DDP training to save GPU memory when training distributed models. Initial tests see a nice reduction of GPU memory used indeed!

This follows the steps of the [main example](https://github.com/facebookresearch/fairscale/blob/master/benchmarks/oss.py) provided on the FairScale repo, integrating them in our Trainer API. To activate training with shared DDP, one must pass along the flag `--sharded_ddp` in a distributed launch command.

Benchmarks tried:
- a fine-tuning on MRPC with `bert_base_uncased` -> goes from 5GB per GPU to 4GB per GPU with no hurt on accuracy
- a fine-tuning on SQUAD v2 with `xlnet_large-cased` -> goes from 11.5GB per GPU to 8GB per GPU (didn't go until the end so didn't check if the accuracy was the same. Training loss seemed equivalent.)
wrt your notes on GPU memory consumption improvements - from what I have seen checking GPU allocation often doesn't show the real difference, as pytorch tends to use more than it absolutely needs if there is spare memory - or rather it can go with less when the memory is tight - so to get the best improvements stats it's the best to try to push instead the BS until it OOMs, and then you get a more precise difference - which usually leads to more precise improvement numbers than just comparing memory allocation. This is just in my experience.

All I'm saying is that probably the improvements are even better than what they seem.<|||||>finetune_trainer crashes with this option:

```
export BS=4; rm -r output_dir; CUDA_VISIBLE_DEVICES=0,1 PYTHONPATH=../../src USE_TF=0   python -m torch.distributed.launch --nproc_per_node=2  ./finetune_trainer.py --model_name_or_path sshleifer/distill-mbart-en-ro-12-4 --output_dir output_dir --adam_eps 1e-06 --data_dir wmt_en_ro --do_train --fp16 --freeze_embeds --label_smoothing 0.1 --learning_rate 3e-5 --logging_first_step --logging_steps 1000 --max_source_length 128 --max_target_length 128 --num_train_epochs 1 --overwrite_output_dir --per_device_train_batch_size $BS --sortish_sampler --src_lang en_XX --task translation --tgt_lang ro_RO --val_max_target_length 128 --warmup_steps 500 --n_train 500 --sharded_ddp
```
```
Traceback (most recent call last):
 File "./finetune_trainer.py", line 379, in <module>
   main()
 File "./finetune_trainer.py", line 315, in main
   trainer.train(
 File "/mnt/nvme1/code/huggingface/transformers-master/src/transformers/trainer.py", line 677, in train
   model = ShardedDDP(model, self.optimizer)
 File "/home/stas/anaconda3/envs/main-38/lib/python3.8/site-packages/fairscale/nn/data_parallel/sharded_ddp.py", line 96, in __init__
   self._param_iterator = chain(*[optim.should_bucket_param.keys() for optim in self.sharded_optimizers])
TypeError: 'AdamW' object is not iterable
```

could probably extend `test_finetune_trainer.py` to deploy this option if `fairscale` is available? but CIs won't have it - and it's quite slow to build
<|||||>Oh it's just because it overrides the `create_optimizer_and_scheduler` method. Will fix that method.<|||||>OK, next we have this:
```
Traceback (most recent call last):
  File "./finetune_trainer.py", line 379, in <module>
    main()
  File "./finetune_trainer.py", line 315, in main
    trainer.train(
  File "/mnt/nvme1/code/huggingface/transformers-master/src/transformers/trainer.py", line 818, in train
    self.scaler.step(self.optimizer)
  File "/home/stas/anaconda3/envs/main-38/lib/python3.8/site-packages/torch/cuda/amp/grad_scaler.py", line 330, in step
    assert len(optimizer_state["found_inf_per_device"]) > 0, "No inf checks were recorded for this optimizer."
AssertionError: No inf checks were recorded for this optimizer.
```

Coincidentally I have just had the same issue with deepspeed integration when I enable its internal fp16 handling. Didn't get to the root of it yet, but removing `--fp16` arg and thus disabling all the fp16 handling trainer does removed this error.

note: I'm switching to deepspeed fp16 handling there...
<|||||>Is it FP16 with AMP or with apex? I don't believe fairscale is compatible with apex.<|||||>native amp

See the command line I'm testing with at:
https://github.com/huggingface/transformers/pull/9139#issuecomment-745581491<|||||>If you're joining in and discovered you can't build `fairscale`, please see [this](https://github.com/facebookresearch/fairscale/pull/249) and perhaps [that](https://github.com/facebookresearch/fairscale/issues/250).<|||||>> OK, next we have this:
> 
> ```
> Traceback (most recent call last):
>   File "./finetune_trainer.py", line 379, in <module>
>     main()
>   File "./finetune_trainer.py", line 315, in main
>     trainer.train(
>   File "/mnt/nvme1/code/huggingface/transformers-master/src/transformers/trainer.py", line 818, in train
>     self.scaler.step(self.optimizer)
>   File "/home/stas/anaconda3/envs/main-38/lib/python3.8/site-packages/torch/cuda/amp/grad_scaler.py", line 330, in step
>     assert len(optimizer_state["found_inf_per_device"]) > 0, "No inf checks were recorded for this optimizer."
> AssertionError: No inf checks were recorded for this optimizer.
> ```
> 
> Coincidentally I have just had the same issue with deepspeed integration when I enable its internal fp16 handling. Didn't get to the root of it yet, but removing `--fp16` arg and thus disabling all the fp16 handling trainer does removed this error.
> 
> note: I'm switching to deepspeed fp16 handling there...

hey there, a bit late, but one of the fairscale/shardedDDP author. The issue with Apex (and vanilla Torch) grad scaler is that it does not know about the gradient sharding, so not all the ranks will have the same behaviour. Torch AMP is supported though, you just have to pass in the ShardedGradScaler as defined here https://github.com/facebookresearch/fairscale/blob/master/fairscale/optim/grad_scaler.py#L24<|||||>Yes, we're passing that scaler :-) The issue was with AMP not Apex. It looks like there is a problem with or without FP16 with one of models.
Ah reading more, I see there is a lot on the issue I posted so will look there. Thanks for coming helping us!