Deprecate Trainer.training_type_plugin in favor of trainer.strategy

[four4fish]

What does this PR do?

follow up for #11120 will rebase after it get merged

part of the clean up work for #10416

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[codecov]

Codecov Report

Merging #11141 (3189996) into master (17ad1a4) will increase coverage by 0%.
The diff coverage is 87%.

@@           Coverage Diff           @@
##           master   #11141   +/-   ##
=======================================
  Coverage      88%      88%           
=======================================
  Files         177      177           
  Lines       16760    16764    +4     
=======================================
+ Hits        14778    14782    +4     
  Misses       1982     1982           

[carmocca]

LGTM.

Some commands that you might find useful for follow-ups

$ grep -iIrn "ttp" pytorch_lightning tests | grep -vi "http"

$ grep -iIrn "training_type" pytorch_lightning tests

[four4fish]

training_type

@carmocca Good call! Updated ttp and training_type words. The training_type pass in import will be updated when we change the file name

[tchaton]

LGTM ! Nice progress.