Introduce ckpt_path="hpc" keyword for checkpoint loading

[otaj]

What does this PR do?

Implements first two points from #13773 (comment)

Does your PR introduce any breaking changes? If yes, please list them.

HPC checkpoints are now NOT being loaded first as a default. Instead, they are specifically loaded if a user passes keyword "hpc". However, if a user is running with SLURMEnvironment and passes ckpt_path=None (which is a default value) and an HPC checkpoint is found, then it is automatically selected.

I'm not sure if we need more tests around this, if yes, I will implement them

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cc @Borda @awaelchli @ananthsub @ninginthecloud @rohitgr7 @otaj @justusschock @akihironitta

[otaj]

Those pesky docstrings 😂. I updated them, @carmocca, let me know if something more should be said there.

[tchaton]

LGTM !