Replace/deprecate SLURM specific checkpointing

[tarepan]

🚀 Feature

Replace HPC/SLURM-specific checkpointing with general checkpointing in CheckpointConnector, then deprecate it.

Motivation

CheckpointConnector has HPC/SLURM-specific checkpointing (save/load system) for auto-resubmit (doc).
Now that auto-resubmit is supported in normal checkpointing process (#4402), HPC auto-resubmit also can be handled by this general process.
In my opinion, the HPC-specific checkpointing ended its historical role.
By deprecating this specific checkpointing, CheckpointConnector can be refactored so simple and become easy to maintain.

Pitch

Deprecate hpc_save & hpc_load, which use hpc_ckpt_{ckpt_number}.ckpt name convention for auto-resume/resubmit.
Use general checkpointing, which attempt to use last.ckpt automatically, for SLURM auto-resubmit.

Backward compatibility

The deprecation break previously-generated checkpoint for auto-resubmit.
But auto-resubmit checkpoint is, in general, used within short-term.
In other words, the checkpoint is ephemeral.

And hpc_save and hpc_load are internal method (no public API in docs).

In this point of views, in my opinion, we can deprecate (internal) old checkpointing without deprecation warning/term.

[tarepan]

Are there any opinion, contributors?
If core contributors agree, I will make PR for this issue.

[edenlightning]

@PyTorchLightning/core-contributors ?

[carmocca]

The idea looks good to me overall.

This issue is pretty much a duplicate of #6204

Closing this one even though it was created first since it contains less activity