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* [fix] Add option to wrap root module in auto_wrap

* Fix unit-test comment

* adding a few more tests to make expected behavior clear

* move changes to wrap policy as suggested

* set default to false

* revert pre-commit change

* revert pre-commit change 2
Co-authored-by: Ruan Silva <ruanrms@fb.com>

* [feat] FSDP: supporting multiple flatten parameter groups

- step 3: make FSDP use FlattenParamModule unconditionally

* fixing the auto_wrap tests

* minor

* rewrite local_metadata_dict

- updated FPW so that custom flat param name is also supported

* bug fix

* mypy

* rewrote consolidate_shard_weights

- test_consolidate passes

* comments

* fixing pickling

* Fix shared params and MoE logic (#749)

* add strict kwarg to support fairseq:gshard MoE saving logic

* Test fairseq style shard

* style

* formatting and address comments

* added changelog

* fixing a test after padding renaming
Co-authored-by: Min Xu <min.xu.public@gmail.com>
Co-authored-by: Sam Shleifer <sshleifer@gmail.com>

* [fix]: handle inputs with containers

- this is an issue surfaces by vissl as well
- fix seems to be super simple
- also cleaned up two tests with respect to multiple such tests
  running back to back (they don't do that presently)

* cleanup

* fix

* lint

* [refactor] enhance wrap and auto_wrap

- Two things were done in this PR
  1. We don't need to import FSDP in wrap.py since the wrapper class
     type is stored in the context now.
  2. We can use a `auto_wrap_policy` function to customize wrapping policy
     for auto_wrap, including size of module, blacklist, exclude list
- The auto_wrap function got simplified a bit as a minor side effect.

* Update fairscale/nn/wrap/auto_wrap.py
Co-authored-by: Sean Naren <sean@grid.ai>

* addressed comments

* addressed more comments
Co-authored-by: Sean Naren <sean@grid.ai>

This adds a context manager that assists in making child modules with similar defaults.
Usage:
```
from fairscale.nn.misc import enable_wrap, wrap

with enable_wrap(**handleful_of_important_params):
    layer_1 = wrap(torch.nn.Linear(5, 5))
    layer_2 = wrap(torch.nn.Linear(5, 5), flatten_parameters=True) # Override parameters if you'd like

# without the context manager, creates Linear layer
layer_1 = wrap(torch.nn.Linear(5, 5))
```
If not within the FSDP context, this would be a no-op. This makes it easier to annotate layers without having to copy any changes in parameters.