Fix checkpoint callback & Trainer.test(_) issue for TPUs

[kaushikb11]

What does this PR do?

Fix checkpoint callback issue for TPUs when set False & `trainer.test(..).
Fixes #6230

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[ananthsub]

what happens if there are multiple checkpoint callbacks attached? should we save once per path?

@awaelchli @carmocca this is gonna be amplified if people are tracking multiple versions of "best model paths" at the same time in an example like this

trainer = Trainer(...., callbacks=[checkpoint1, checkpoint2])
trainer.fit(module)
trainer.test()  <--- what checkpoint path path is used for running this?

should this raise an error due to ambiguity?

[carmocca]

I think I'd rather use the first best path and log the path used when running test

[tchaton]

LGTM !

[SeanNaren]

Nice! on a side note how far are we from enabling TPU tests again?

[kaushikb11]

@SeanNaren Thanks for bringing it up! We could keep a look on it for next couple of days, I have seen instances when the TPU Pods used to get killed mid testing. Afk, will create an Issue and track it.

[Borda]

have you tested it locally as TPU is put for now...?

[kaushikb11]

@Borda

All tests are passing. Had to do a tweek. Will investigate it soon.

[codecov]

Codecov Report

Merging #6654 (4c69b62) into master (d471fa3) will decrease coverage by 5%.
The diff coverage is 29%.

@@           Coverage Diff           @@
##           master   #6654    +/-   ##
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- Coverage      91%     86%    -5%     
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  Files         192     192            
  Lines       12227   12373   +146     
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- Hits        11111   10654   -457     
- Misses       1116    1719   +603