[FSDP] Adding Native FSDP Strategy

[sisilmehta2000]

What does this PR do?

Pytorch 1.11.0 makes torch.distributed.fsdp available for public use (PR #72084) https://github.com/pytorch/pytorch/releases/tag/v1.11.0

Currently there is a strategies/fully_sharded.py strategy that uses the fairscale implementation of fully sharded data parallel technique. With the introduction of fsdp in torch.distributed we want to create a new strategy called strategies/fully_sharded2.py

Reasons to add a new strategy:

• Torch.distributed.fsdp doesn’t currently implement all the features of the fairscale version
• We don't want the torch distributed API to feel restricted based on current integrations with fairscale. By keeping them separate, we'll adapt to whatever recommendation the torch.distributed team provides
• It's cheap/easy for us to add a new strategy class
• We want to have people start using the new torch.distributed.fsdp version without introducing a breaking change to those using the fairscale version

Fixes #12334

Does your PR introduce any breaking changes? If yes, please list them.

None

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[sisilmehta2000]

So some of the tests like conda(1.0, 1.8) are failing because it can't find torch.distributed.fsdp. That only exists in torch version 1.11.

[Borda]

goooo 🎉