Deprecate amp_level from Trainer

[rohitgr7]

What does this PR do?

Fixes #12655

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

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Make sure you had fun coding 🙃

cc @tchaton @rohitgr7 @carmocca @justusschock @awaelchli @akihironitta @kaushikb11 @Borda

[codecov]

Codecov Report

Merging #13898 (565bada) into master (332182d) will increase coverage by 15%.
The diff coverage is 100%.

❗ Current head 565bada differs from pull request most recent head 8065f5c. Consider uploading reports for the commit 8065f5c to get more accurate results

@@            Coverage Diff            @@
##           master   #13898     +/-   ##
=========================================
+ Coverage      61%      76%    +15%     
=========================================
  Files         341      341             
  Lines       26596    26608     +12     
=========================================
+ Hits        16211    20247   +4036     
+ Misses      10385     6361   -4024     

[carmocca]

LGTM other than the removal version

[akihironitta]

LGTM!