Fixed num_sanity_val_steps affecting reproducibility of training data shuffling

[awaelchli]

What does this PR do?

Fixes #6988

Test fails on master

Important note
This fix will impact reproducibility from one version to the next as the trainer runs a number of sanity validation steps by default.

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[codecov]

Codecov Report

Merging #7014 (89fa68e) into master (6be0a85) will decrease coverage by 1%.
The diff coverage is 83%.

@@           Coverage Diff           @@
##           master   #7014    +/-   ##
=======================================
- Coverage      91%     91%    -1%     
=======================================
  Files         198     198            
  Lines       12634   12790   +156     
=======================================
+ Hits        11541   11598    +57     
- Misses       1093    1192    +99     

[awaelchli]

windows problems and macOS timing out :)

[tchaton]

LGTM !

[kaushikb11]

LGTM! But there seems to be a failing test