Wct save interval

[KuuCi]

What does this PR do?

This PR attempts to support wall clock time as a interval metric. Users will be able to set wall clock time in training yamls to define their save interval. (https://docs.google.com/document/d/1zEs6YKN5YpnwQD-CXjyHlW7R98B3P8ORjEIqA5D8iwI/edit). Specifically, it introduces TimeUnit.SECOND, TimeUnit.MINUTE, TimeUnit.HOUR while keeping all time arithmetic in seconds (minutes and hours are converted to seconds).

manual test: wct-test-sec-v8msnl, wct-test-min-oUF2KU, wct-test-hr-A5zIE0

Checkpoints:
https://cloud.oracle.com/object-storage/buckets/axhe5a72vzpp/vincent-test/objects?region=us-phoenix-1
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What issue(s) does this change relate to?

N/A

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[mvpatel2000]

Please provide a PR description!

[mvpatel2000]

I thought we were going to use train timestamp?

[KuuCi]

Working: wct-test-2m-UAMkGI
Broken Scheduler: wct-test-2m-xJkqik
Broken Max Duration: wct-test-2m-NupAgq

[mvpatel2000]

A few nits

[mvpatel2000]

Mostly LGTM! Final nit :)

[mvpatel2000]

LGTM!