Removed deprecated Trainer.num_processes property in favour of Trainer.num_devices

[krishnakalyan3]

What does this PR do?

Deprecate APIs

resolves #12388

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[codecov]

Codecov Report

Merging #14423 (7ac851c) into master (1e04951) will increase coverage by 15%.
The diff coverage is n/a.

❗ Current head 7ac851c differs from pull request most recent head d4e75f2. Consider uploading reports for the commit d4e75f2 to get more accurate results

@@            Coverage Diff            @@
##           master   #14423     +/-   ##
=========================================
+ Coverage      61%      76%    +15%     
=========================================
  Files         331      332      +1     
  Lines       26650    26903    +253     
=========================================
+ Hits        16357    20438   +4081     
+ Misses      10293     6465   -3828     

[krishnakalyan3]

Thanks for the reviews @rohitgr7 and @awaelchli