Add MpModelWrapper in TPU Spawn

[kaushikb11]

What does this PR do?

Add MpModelWrapper in TPU Spawn.

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[pep8speaks]

Hello @kaushikb11! Thanks for updating this PR.

There are currently no PEP 8 issues detected in this Pull Request. Cheers! 🍻

Comment last updated at 2021-04-20 12:04:48 UTC

[kaushikb11]

Converted to Draft, as tpu tests were failing.

[tchaton]

This should be wrapped_model.

[kaushikb11]

Assigning it while moving the model to device. https://github.com/PyTorchLightning/pytorch-lightning/blob/tpu_spawn_added/pytorch_lightning/plugins/training_type/tpu_spawn.py#L172

MpModelWrapper is just using mp lock and returns the model https://github.com/pytorch/xla/blob/master/torch_xla/distributed/xla_multiprocessing.py#L432