Avoid torch amp cuda warning with bf16 on cpu

[rohitgr7]

What does this PR do?

Use precision_plugin context manager to avoid following warning with bf16 on CPU.

UserWarning: User provided device_type of 'cuda', but CUDA is not available. Disabling

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Part of #1

cc @Borda @carmocca @justusschock @awaelchli @akihironitta

[github-actions]

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

[awaelchli]

nice to see this being generalized.

[codecov]

Codecov Report

Merging #11161 (16ebbc6) into master (7ed3dbf) will decrease coverage by 0%.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master   #11161   +/-   ##
=======================================
- Coverage      92%      92%   -0%     
=======================================
  Files         177      177           
  Lines       16556    16554    -2     
=======================================
- Hits        15223    15213   -10     
- Misses       1333     1341    +8