GitLab

* [FSDP] Add an arg for FSDP __init__

Add an arg, disable_reshard_on_root, for FSDP __init__ to handle the following issue
https://github.com/facebookresearch/fairscale/issues/878


For some cases (models wrapped by autowrap), the parameters (of root modules) needs to be sharded, and reshard_after_forward should not be set to False.
"disable_reshard_on_root" is for users to choose whether to force reshard_after_forward of root modules to be False or not.

* Update fully_sharded_data_parallel.py

Modified the description of the feature to explain more clear.

* Update fairscale/nn/data_parallel/fully_sharded_data_parallel.py

Update the comments for disable_reshard_on_root
Co-authored-by: Min Xu <24926999+min-xu-ai@users.noreply.github.com>

* Modified the comments

Modified the comments of disable_reshard_on_root
Co-authored-by: Min Xu <24926999+min-xu-ai@users.noreply.github.com>