Refactor XLA and TPU checks across codebase

[carmocca]

What does this PR do?

Move XLA utilities together with the TPUAccelerator
Deprecate one more redundant utility function
Fix mocks in tests via fixtures

Follow-up to #14514
Part of #11826
Part of #12786
Closes #14982, #14979

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

None

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cc @Borda @justusschock @awaelchli @rohitgr7 @tchaton

[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.

[codecov]

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     

[tchaton]

LGTM !