Handle collision of user argument when using ShardedDDP

[SeanNaren]

What does this PR do?

Ensure that if a user passes reduce_buffer_size manually, we handle this case appropriately, i.e:

plugin = DDPShardedPlugin(reduce_buffer_size=1e8)

Related #9483

cc @ananthsub

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[tchaton]

LGMT !

[codecov]

Codecov Report

Merging #9512 (779abcf) into master (c784092) will increase coverage by 0%.
The diff coverage is 100%.

@@          Coverage Diff           @@
##           master   #9512   +/-   ##
======================================
  Coverage      93%     93%           
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  Files         180     180           
  Lines       15072   15074    +2     
======================================
+ Hits        13997   13999    +2     
  Misses       1075    1075