Fix Checkpoint issue when using Horovod distributed backend (PyTorchLightning#6947)

[liob]

What does this PR do?

Fixes #6947: ModelCheckpoint failed when using the save_top_k feature because the ReduceOp was not considered when evaluating reduce_sum.

cc @awaelchli

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[codecov]

Codecov Report

Merging #6958 (f10b2a4) into master (6c01608) will increase coverage by 44%.
The diff coverage is 0%.

❗ Current head f10b2a4 differs from pull request most recent head 7d32ceb. Consider uploading reports for the commit 7d32ceb to get more accurate results

@@           Coverage Diff            @@
##           master   #6958     +/-   ##
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+ Coverage      43%     87%    +44%     
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  Files         194     194             
  Lines       12365   12346     -19     
========================================
+ Hits         5330   10781   +5451     
+ Misses       7035    1565   -5470