Refactor `XLADeviceUtils`

[carmocca]

What does this PR do?

Move XLA utilities together with the TPUAccelerator Deprecate one more redundant utility function

Follow-up to https://github.com/Lightning-AI/lightning/pull/14514 Part of #11826 Part of #12786

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

None

Before submitting

• [x] Was this discussed/approved via a GitHub issue? (not for typos and docs)
• [x] Did you read the contributor guideline, Pull Request section?
• [x] Did you make sure your PR does only one thing, instead of bundling different changes together?
• [n/a] Did you make sure to update the documentation with your changes? (if necessary)
• [x] Did you write any new necessary tests? (not for typos and docs)
• [x] Did you verify new and existing tests pass locally with your changes?
• [x] Did you list all the breaking changes introduced by this pull request?
• [x] Did you update the CHANGELOG? (not for typos, docs, test updates, or minor internal changes/refactors)

PR review

• [ ] Is this pull request ready for review? (if not, please submit in draft mode)
• [x] Check that all items from Before submitting are resolved
• [x] Make sure the title is self-explanatory and the description concisely explains the PR
• [x] Add labels and milestones (and optionally projects) to the PR so it can be classified

[awaelchli]

@carmocca just to be sure, you are not mixing up tpu_available with xla_available? These are two different things. One can have xla available on a machine without tpus.

[carmocca]

@awaelchli In this PR, tpu_available is deleted in favor of TPUAccelerator.is_available().

In most places, we want to check XLA_AVAILABLE as that's what needs to be True to successfully import torch_xla. TPUAccelerator.is_available() includes XLA_AVAILABLE because torch_xla is necessary to check whether we have TPU hardware.

I don't think I mixed things up, but there might be mistakes. Tests are not fixed yet.

[carmocca]

@awaelchli I'll need you to run MPS tests locally with this branch

[codecov[bot]]

Codecov Report

Merging #14550 (d296b7b) into master (b936fd4) will increase coverage by 2%. The diff coverage is 66%.

Additional details and impacted files

@@            Coverage Diff            @@
##           master   #14550     +/-   ##
=========================================
+ Coverage      83%      85%     +2%     
=========================================
  Files         396      275    -121     
  Lines       29007    21166   -7841     
=========================================
- Hits        24032    17919   -6113     
+ Misses       4975     3247   -1728