update FSDP and add XLA-FSDP documentation (#21812)

* update FSDP and add XLA-FSDP documentation

* resolving comments

* minor update

* fix xla-fsdp docs

docs/source/en/main_classes/trainer.mdx

@@ -569,27 +569,64 @@ as the model saving with FSDP activated is only available with recent fixes.
     For this, add `--fsdp shard_grad_op` to the command line arguments.
   - NO_SHARD : No sharding. For this, add `--fsdp no_shard` to the command line arguments.
 - To offload the parameters and gradients to the CPU, 
-add `--fsdp "full_shard offload"` or `--fsdp "shard_grad_op offload"` to the command line arguments.
+  add `--fsdp "full_shard offload"` or `--fsdp "shard_grad_op offload"` to the command line arguments.
 - To automatically recursively wrap layers with FSDP using `default_auto_wrap_policy`, 
-add `--fsdp "full_shard auto_wrap"` or `--fsdp "shard_grad_op auto_wrap"` to the command line arguments.
+  add `--fsdp "full_shard auto_wrap"` or `--fsdp "shard_grad_op auto_wrap"` to the command line arguments.
 - To enable both CPU offloading and auto wrapping, 
-add `--fsdp "full_shard offload auto_wrap"` or `--fsdp "shard_grad_op offload auto_wrap"` to the command line arguments.
-- If auto wrapping is enabled, you can either use transformer based auto wrap policy or size based auto wrap policy.
-  - For transformer based auto wrap policy, please add `--fsdp_transformer_layer_cls_to_wrap <value>` to command line arguments.
-  This specifies the transformer layer class name (case-sensitive) to wrap ,e.g, `BertLayer`, `GPTJBlock`, `T5Block` ....
+  add `--fsdp "full_shard offload auto_wrap"` or `--fsdp "shard_grad_op offload auto_wrap"` to the command line arguments.
+- Remaining FSDP config is passed via `--fsdp_config <path_to_fsdp_config.json>`. It is either a location of
+  FSDP json config file (e.g., `fsdp_config.json`) or an already loaded json file as `dict`. 
+  - If auto wrapping is enabled, you can either use transformer based auto wrap policy or size based auto wrap policy.
+    - For transformer based auto wrap policy, please specify `fsdp_transformer_layer_cls_to_wrap` in the config file. 
+      This specifies the list of transformer layer class name (case-sensitive) to wrap ,e.g, [`BertLayer`], [`GPTJBlock`], [`T5Block`] ....
       This is important because submodules that share weights (e.g., embedding layer) should not end up in different FSDP wrapped units.
       Using this policy, wrapping happens for each block containing Multi-Head Attention followed by couple of MLP layers. 
       Remaining layers including the shared embeddings are conveniently wrapped in same outermost FSDP unit.
       Therefore, use this for transformer based models.
-  - For size based auto wrap policy, please add `--fsdp_min_num_params <number>` to command line arguments.
+    - For size based auto wrap policy, please add `fsdp_min_num_params` in the config file. 
       It specifies FSDP's minimum number of parameters for auto wrapping.
+  - `fsdp_backward_prefetch` can be specified in the config file. It controls when to prefetch next set of parameters. 
+    `backward_pre` and `backward_pos` are available options. 
+    For more information refer `torch.distributed.fsdp.fully_sharded_data_parallel.BackwardPrefetch`
+  - `fsdp_forward_prefetch` can be specified in the config file. It controls when to prefetch next set of parameters. 
+    If `"True"`, FSDP explicitly prefetches the next upcoming all-gather while executing in the forward pass. 
+  - `limit_all_gathers` can be specified in the config file. 
+    If `"True"`, FSDP explicitly synchronizes the CPU thread to prevent too many in-flight all-gathers.
 
 **Few caveats to be aware of**
-- Mixed precision is currently not supported with FSDP as we wait for PyTorch to fix support for it.
-More details in this [issues](https://github.com/pytorch/pytorch/issues/75676).
-- FSDP currently doesn't support multiple parameter groups. 
-More details mentioned in this [issue](https://github.com/pytorch/pytorch/issues/76501)
-(`The original model parameters' .grads are not set, meaning that they cannot be optimized separately (which is why we cannot support multiple parameter groups)`).
+- it is incompatible with `generate`, thus is incompatible with `--predict_with_generate` 
+  in all seq2seq/clm scripts (translation/summarization/clm etc.).  
+  Please refer issue [#21667](https://github.com/huggingface/transformers/issues/21667)
+
+### PyTorch/XLA Fully Sharded Data parallel
+
+For all the TPU users, great news! PyTorch/XLA now supports FSDP.
+All the latest Fully Sharded Data Parallel (FSDP) training are supported.
+For more information refer to the [Scaling PyTorch models on Cloud TPUs with FSDP](https://pytorch.org/blog/scaling-pytorch-models-on-cloud-tpus-with-fsdp/) and [PyTorch/XLA implementation of FSDP](https://github.com/pytorch/xla/tree/master/torch_xla/distributed/fsdp)
+All you need to do is enable it through the config.
+
+**Required PyTorch/XLA version for FSDP support**: >=2.0
+
+**Usage**:
+
+Pass `--fsdp "full shard"` along with following changes to be made in `--fsdp_config <path_to_fsdp_config.json>`:
+- `xla` should be set to `True` to enable PyTorch/XLA FSDP.
+- `xla_fsdp_settings` The value is a dictionary which stores the XLA FSDP wrapping parameters.
+  For a complete list of options, please see [here](
+  https://github.com/pytorch/xla/blob/master/torch_xla/distributed/fsdp/xla_fully_sharded_data_parallel.py).
+- `xla_fsdp_grad_ckpt`. When `True`, uses gradient checkpointing over each nested XLA FSDP wrapped layer. 
+  This setting can only be used when the xla flag is set to true, and an auto wrapping policy is specified through
+  `fsdp_min_num_params` or `fsdp_transformer_layer_cls_to_wrap`. 
+- You can either use transformer based auto wrap policy or size based auto wrap policy.
+  - For transformer based auto wrap policy, please specify `fsdp_transformer_layer_cls_to_wrap` in the config file. 
+    This specifies the list of transformer layer class name (case-sensitive) to wrap ,e.g, [`BertLayer`], [`GPTJBlock`], [`T5Block`] ....
+    This is important because submodules that share weights (e.g., embedding layer) should not end up in different FSDP wrapped units.
+    Using this policy, wrapping happens for each block containing Multi-Head Attention followed by couple of MLP layers. 
+    Remaining layers including the shared embeddings are conveniently wrapped in same outermost FSDP unit.
+    Therefore, use this for transformer based models.
+  - For size based auto wrap policy, please add `fsdp_min_num_params` in the config file. 
+    It specifies FSDP's minimum number of parameters for auto wrapping.
+
 
 ### Using Trainer for accelerated PyTorch Training on Mac