Improve code quality in AcceleratorConnector._configure_slurm_ddp

[awaelchli]

What does this PR do?

Follow up to #10101
Part of #7740 #10417 (simplifying accelerator connector) (baby steps)

Simplifies the _configure_slurm_ddp method and improves code quality. Adds a docstring for explanation.

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Did you have fun?

I made sure I had fun coding 🙃

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

cc @Borda @justusschock @awaelchli @akihironitta

[kaushikb11]

Nice essential clean up!

[tchaton]

LGTM !

[codecov]

Codecov Report

Merging #10102 (2e005a8) into master (c15b84d) will increase coverage by 49%.
The diff coverage is 100%.

❗ Current head 2e005a8 differs from pull request most recent head 4129e9e. Consider uploading reports for the commit 4129e9e to get more accurate results

@@            Coverage Diff            @@
##           master   #10102     +/-   ##
=========================================
+ Coverage      43%      92%    +49%     
=========================================
  Files         178      178             
  Lines       16244    16240      -4     
=========================================
+ Hits         6936    14900   +7964     
+ Misses       9308     1340   -7968