Move DP warning suppression to the DataParallel override

[ananthsub]

What does this PR do?

• Simplifies what's under pytorch_lightning/trainer/
• Since this is specific to DataParallel, we should include it as part of the plugin definition

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[codecov]

Codecov Report

Merging #7421 (ace2fca) into master (fecce50) will decrease coverage by 5%.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master   #7421    +/-   ##
=======================================
- Coverage      92%     87%    -5%     
=======================================
  Files         200     199     -1     
  Lines       12978   12978            
=======================================
- Hits        11942   11354   -588     
- Misses       1036    1624   +588     

[carmocca]

This warning is produced by torch, correct?

[awaelchli]

amazing!