Set find unused parameters to True by default to fix breaking compatibility

[SeanNaren]

What does this PR do?

Fixes #6219
Fixes #5799
Fixes #6409
Fixes #5604

A lot of back and forth here, but we're making a trade off for compatability/speed. This PR suggests setting the default to True, and adding into the documentation to set this parameter to False for speed.

As always I suggest checking Sharded Training, which does not have the same issue!

cc @ananthsub @ericharper @tchaton @carmocca @awaelchli @justusschock

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[codecov]

Codecov Report

Merging #6438 (5070a49) into master (55dd3a4) will not change coverage.
The diff coverage is n/a.

@@          Coverage Diff           @@
##           master   #6438   +/-   ##
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  Coverage      94%     94%           
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  Files         161     161           
  Lines       11495   11495           
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  Hits        10751   10751           
  Misses        744     744           

[awaelchli]

I linked two more issues