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update fsdp docs #18521

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Aug 8, 2022
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update fsdp docs #18521

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b62e925
updating fsdp documentation
pacman100 Aug 8, 2022
ae12231
typo fix
pacman100 Aug 8, 2022
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12 changes: 10 additions & 2 deletions docs/source/en/main_classes/trainer.mdx
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Expand Up @@ -567,14 +567,22 @@ as the model saving with FSDP activated is only available with recent fixes.
For this, add `--fsdp full_shard` to the command line arguments.
- SHARD_GRAD_OP : Shards optimizer states + gradients across data parallel workers/GPUs.
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.
- 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.
- 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, please add `--fsdp_min_num_params <number>` to command line arguments.
It specifies FSDP's minimum number of parameters for Default Auto Wrapping.
- 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` ....
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 conviniently 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.
It specifies FSDP's minimum number of parameters for auto wrapping.

**Few caveats to be aware of**
- Mixed precision is currently not supported with FSDP as we wait for PyTorch to fix support for it.
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