Add support for skipping validation on resume + extend saving last ckpt test
# What does this PR do ?

When restarting training from a saved checkpoint during validation, validation is re-run as a first step (as the checkpoint was saved just before validation). This PR introduces a new loop injected via the `exp_manager` that skips validation if we're restarting.

There was also a mistake with the test definition from #4905 and I extended the test further to check @yaoyu-33s' case.

cc @MaximumEntropy @ericharper @titu1994 

# Changelog 
- Skip initial validation by default when resuming from checkpoint which saved after a validation pass has occurred.

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**PR Type**:
- [ ] New Feature
- [x] Bugfix
- [ ] Documentation

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