implement XLAHooks and register it to PyTorch when loaded.

[iwknow]

This PR adds XLAHooks that override the placeholder implementation added pytorch/pytorch#161369.

only part of the methods are implemented. I believe not all the methods are actually needed. therefore, i only implemented the most basic ones. People can implement the other methods when they need.

IT DOSEN'T BUILD, BUT FEEL FREE TO REVIEW!
This PR depends on pytorch/pytorch#161369 and #9682. This PR won't build before the two PR submitted.

[ysiraichi]

See https://github.com/pytorch/xla/blob/master/.github/ci.md#temporarily-pinning-pytorch-pr-in-pytorchxla-pr

[ysiraichi]

Just a heads up: you are probably going to need to work on #9703 (updating the PyTorch pin) before this one.
Before doing that, check the CI failures in that PR. It might just be that you need land them together: add #9703 changes to this PR.

[iwknow]

i suggest to check in #9703 before submitting this PR and keep the two PR separately. I can take a look at #9703 if you cannot find anyone working on it.

[ysiraichi]

I won't be able to keep working on it. Since you need that for this PR, I was just giving you a heads up.
Let me know if you need any help.