feat: add standalone guide on XLA support.
# What does this PR do?

We have had XLA support for our TF generation models (GPT2, Whisper, for example) for a while. This PR adds a standalone guide in the doc to discuss it. 

Cc: @Rocketknight1 @amyeroberts @ydshieh 
@sayakpaul 

It seems there is an issue with your CircleCI permissions.
Could you try refreshing your permissions as shown [here](https://support.circleci.com/hc/en-us/articles/360048210711-How-to-Refresh-User-Permissions-)?<|||||>@ydshieh I don't have CircleCI signed in nor have I installed it here: https://github.com/settings/applications. <|||||>Could you login CircleCI with your GitHub account, and follow `huggingface/transformers`?

See https://circleci.com/docs/projects/

(but it's kinda strange - you have opened a lot of PRs before, so not sure why we have this issue now)<|||||>Should be all good now I guess:

<img width="896" alt="image" src="https://user-images.githubusercontent.com/22957388/212717447-19e1b042-d39f-44f0-bf8c-faef7b97c2ea.png">
<|||||>Yeah, then we can try to push an empty commit to see if the CI will run :-)
```
git commit --allow-empty -m "Empty commit to trigger CI"
git push
```
<|||||>_The documentation is not available anymore as the PR was closed or merged._<|||||>@sgugger addressed your comments including [this one](https://github.com/huggingface/transformers/pull/21141/files#r1072172489). Let me know if I am good to merge given all tests are green. <|||||>Yes, all good!