fix overfit_batch sampler replacement logic

[rohitgr7]

What does this PR do?

Fixes #10457
The fix is that we check for RandomSampler to identify whether shuffling is happening or not.. instead we should check whether the sampler is SequentialSampler or not.
Also improved some similar tests a little bit

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[codecov]

Codecov Report

Merging #10486 (00294e5) into master (1de3539) will decrease coverage by 0%.
The diff coverage is 100%.

❗ Current head 00294e5 differs from pull request most recent head a31cb42. Consider uploading reports for the commit a31cb42 to get more accurate results

@@           Coverage Diff           @@
##           master   #10486   +/-   ##
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- Coverage      92%      92%   -0%     
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  Files         178      178           
  Lines       16240    16239    -1     
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- Hits        14901    14900    -1     
  Misses       1339     1339           

[tchaton]

LGTM !