Rename TPUHalfPrecisionPlugin to TPUBf16PrecisionPlugin

[carmocca]

What does this PR do?

Fixes #10020 (comment)

Rename TPUHalfPrecisionPlugin to TPUBf16PrecisionPlugin
And prints a warning when the user sets precision=16 for TPUs as they only really support "bf16".

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

The class name has changed

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[github-actions]

Build Error! No Linked Issue found. Please link an issue or mention it in the body using #<issue_id>

[tchaton]

LGTM !

[codecov]

Codecov Report

Merging #10026 (e75171c) into master (e8beceb) will decrease coverage by 4%.
The diff coverage is 100%.

@@           Coverage Diff            @@
##           master   #10026    +/-   ##
========================================
- Coverage      93%      89%    -4%     
========================================
  Files         180      180            
  Lines       15854    15870    +16     
========================================
- Hits        14678    14075   -603     
- Misses       1176     1795   +619     

[edward-io]

Are we worried about backwards compatibility here, or is that primarily around API changes?

[carmocca]

Are we worried about backwards compatibility here, or is that primarily around API changes?

We would if the API was stable.

In the past, we've still added deprecation paths for very popular plugins or hooks. I don't think that's the case with the TPUHalfPrecisionPlugin