Enable DDP Plugin to pass through args to LightningDistributedDataParallel

[ananthsub]

What does this PR do?

This makes the DDP Plugin more generic, so people don't have to subclass if they only want to change what goes into the DistributedDataParallel constructor. This preserves find_unused_parameters=True as the default.

Related: is find_unused_parameters forced to be True since Lightning wraps the whole LightningModule in LightningDistributedDataParallel ?

cc @williamFalcon

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[codecov]

Codecov Report

Merging #4382 into master will increase coverage by 0%.
The diff coverage is 100%.

@@          Coverage Diff           @@
##           master   #4382   +/-   ##
======================================
  Coverage      93%     93%           
======================================
  Files         111     111           
  Lines        8135    8099   -36     
======================================
- Hits         7526    7509   -17     
+ Misses        609     590   -19     

[tchaton]

Awesome addition !