[feat] Support time-based checkpointing during training

[ananthsub]

What does this PR do?

Fixes #6286

To discuss: The PR currently enforces each trigger (every_n_train_steps, train_time_interval, every_n_val_epochs) to be mutually exclusive. This means if someone wants to checkpoint every N hours and every M train batches, they'd need to create two callbacks.

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[codecov]

Codecov Report

Merging #7515 (055ec2a) into master (485554c) will decrease coverage by 4%.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master   #7515    +/-   ##
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- Coverage      92%     88%    -4%     
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  Files         198     198            
  Lines       12912   12930    +18     
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- Hits        11912   11368   -544     
- Misses       1000    1562   +562     

[tchaton]

LGTM !

[SkafteNicki]

Maybe worth adding a note in the case the user wants to do multiple stuff at the same time

.. note:
   If you want a checkpoint at both every N hours and every M train batches or val epochs then multiple ``ModelCheckpoint`` should be created.