Deprecate Trainer.should_rank_save_checkpoint property

[jjenniferdai]

What does this PR do?

see context: #11035 (comment)
original PR: #9433
original issue: #9074

Go through deprecation for public property

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

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[codecov]

Codecov Report

Merging #11068 (13b41f7) into master (06c5903) will decrease coverage by 4%.
The diff coverage is 100%.

@@           Coverage Diff            @@
##           master   #11068    +/-   ##
========================================
- Coverage      92%      88%    -4%     
========================================
  Files         177      177            
  Lines       16492    16502    +10     
========================================
- Hits        15151    14551   -600     
- Misses       1341     1951   +610     

[awaelchli]

I don't understand how we can deprecate this on the trainer while at the same time remove it on the strategy. You will have to copy this implementation into the deprecated property method:

return isinstance(self.ttp, TPUSpawn) and self.local_rank == 0 or self.is_global_zero and it won't cover custom plugins.

So yeah, this will be confusing.