Convert subprocess test to standalone test

[awaelchli]

What does this PR do?

Needed for #13967

In #13967 there are two tests that fail:

• test_multi_gpu_model_ddp_fit_only
• test_multi_gpu_model_ddp_fit_test

These tests are unrelated to deepspeed, yet they fail consistently when I upgrade deepspeed in the CI. It is not reproducible locally and I couldn't figure out the reason. However, these tests are old, and if I convert them to standalone tests like we have for other similar ddp tests, there is no problem.

Hence, I propose to just refactor these old tests into standalone tests to move forward. It would have been done sooner or later anyway.

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• [x] Did you write any new necessary tests? (not for typos and docs)
• [x] Did you verify new and existing tests pass locally with your changes?
• [x] Did you update the CHANGELOG? (not for typos, docs, test updates, or internal minor changes/refactorings)

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Did you have fun?

I made sure I had fun coding 🙃

cc @justusschock @awaelchli @rohitgr7 @borda @akihironitta @kaushikb11

[codecov[bot]]

Codecov Report

Merging #14101 (9ce4cbf) into master (dc8ff5e) will increase coverage by 15%. The diff coverage is n/a.

:exclamation: Current head 9ce4cbf differs from pull request most recent head 25e4f11. Consider uploading reports for the commit 25e4f11 to get more accurate results

@@            Coverage Diff            @@
##           master   #14101     +/-   ##
=========================================
+ Coverage      61%      76%    +15%     
=========================================
  Files         324      326      +2     
  Lines       26408    26572    +164     
=========================================
+ Hits        16144    20227   +4083     
+ Misses      10264     6345   -3919     

[awaelchli]

The test failures here happen on master too, so I'm not going to investigate in this PR.

test_configure_external_callbacks

 TypeError: 'Mock' object is not iterable