[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 #7613 (0f70c81) into master (485554c) will decrease coverage by 0%.
The diff coverage is 100%.

@@          Coverage Diff           @@
##           master   #7613   +/-   ##
======================================
- Coverage      92%     92%   -0%     
======================================
  Files         198     198           
  Lines       12912   12930   +18     
======================================
+ Hits        11912   11914    +2     
- Misses       1000    1016   +16     

[carmocca]

Does this replace #7515?

What's the difference?

[ananthsub]

Does this replace #7515?

What's the difference?

there's no difference. this was following up from our slack discussion as to whether the GPU tests were failing was because i was working off a branch in my fork vs in the lightning repo directly. since the base repo changed, i needed to create a new PR