move fsdp handling to accelerate
### What does this PR do?
1. Moves PyTorch FSDP handling to Accelerate
2. Should be merged after #23151 
3. No user-facing change. Now, users can use `accelerate launch` for fsdp in Trainer, e.g.:
```
accelerate launch --num_processes=2 --use_fsdp --mixed_precision=bf16 --fsdp_auto_wrap_policy=TRANSFORMER_BASED_WRAP  --fsdp_transformer_layer_cls_to_wrap="BertLayer" --fsdp_sharding_strategy=1 --fsdp_state_dict_type=FULL_STATE_DICT ./examples/pytorch/text-classification/run_glue.py   --model_name_or_path bert-base-cased   --task_name $TASK_NAME   --do_train   --do_eval   --max_seq_length 128   --per_device_train_batch_size 16   --learning_rate 5e-5   --num_train_epochs 3   --output_dir /tmp/$TASK_NAME/ --overwrite_output_dir
```

Continue to use torchrun with trainer args as usual.
```
torchrun --nnodes 1 --nproc-per-node 2 ./examples/pytorch/text-classification/run_glue.py   --model_name_or_path bert-base-cased   --task_name $TASK_NAME   --do_train   --do_eval   --max_seq_length 128   --per_device_train_batch_size 16   --learning_rate 5e-5   --num_train_epochs 3   --output_dir /tmp/$TASK_NAME/ --overwrite_output_dir --fsdp "full_shard auto_wrap" --fsdp_transformer_layer_cls_to_wrap BertLayer --bf16
```
_The documentation is not available anymore as the PR was closed or merged._<|||||>Hello Sylvain, we can't do that as FSDP XLA integration uses it and that isn't supported yet in accelerate