Add XLA Dynamo backends for training and inference

[johnsutor]

What does this PR do?

This PR introduces the TorchDynamo integration with PyTorch XLA. Though I haven't thorougly benchmarked the code, it works on the CLM example from the Transformers library, when spawned using the accelerate launch command on a v5e TPU VM with 8 TPUs running on Ubuntu 22.04. It also passes all tests I ran on the TPU.

Fixes #2870

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@SunMarc

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[SunMarc]

Thanks for adding this @johnsutor ! cc @muellerzr for visibility

[muellerzr]

Nice! Thanks for enabling this :)