Mark SLURM detection methods in AcceleratorConnector as protected

[awaelchli]

What does this PR do?

Marks two SLURM related APIs as protected.

• AcceleratorConnector.is_slurm_managing_tasks: This is an internal bool used to select the right plugins. A user should check isinstance(trainer.cluster_environment, SLURMEnvironment) instead.
• AcceleratorConnector.configure_slurm_ddp: This is an internal method used to set bools and select the right plugin for cluster management. A user is not meant to interact directly with this method.

I propose a short deprecation phase of 1.5-1.6 because these public methods are likely not being used by anyone.

Part of #9778
Part of #6303

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Part of #1 (it's a lie, this is just here to avoid noisy GitHub bot)

[daniellepintz]

if these are both internal methods, could we just skip deprecation cycle?

[tchaton]

LGTM !

[awaelchli]

@daniellepintz If this goes into 1.5 then for us the cost will be almost 0. I marked it for removal in 1.6 so we can remove it immediately on master.

[codecov]

Codecov Report

Merging #10101 (c1f65c9) into master (d3e5a43) will decrease coverage by 4%.
The diff coverage is 95%.

@@           Coverage Diff            @@
##           master   #10101    +/-   ##
========================================
- Coverage      92%      89%    -4%     
========================================
  Files         182      182            
  Lines       16153    16164    +11     
========================================
- Hits        14940    14332   -608     
- Misses       1213     1832   +619