Split trainer cli function into two functions

[roomrys]

Description

I split the main training function into two (for easier testing).

Types of changes

• Bugfix
• New feature
• Refactor / Code style update (no logical changes)
• Build / CI changes
• Documentation Update
• Other (explain)

Does this address any currently open issues?

• Training subprocess has returncode of 1 even though successful training #1193

Outside contributors checklist

• Review the guidelines for contributing to this repository
• Read and sign the CLA and add yourself to the authors list
• Make sure you are making a pull request against the develop branch (not main). Also you should start your branch off develop
• Add tests that prove your fix is effective or that your feature works
• Add necessary documentation (if appropriate)

Thank you for contributing to SLEAP!

❤️

[codecov]

Codecov Report

Merging #1197 (21436fe) into develop (1c7b314) will decrease coverage by 0.50%.
The diff coverage is 50.00%.

@@             Coverage Diff             @@
##           develop    #1197      +/-   ##
===========================================
- Coverage    72.32%   71.82%   -0.50%     
===========================================
  Files          132      132              
  Lines        23326    23328       +2     
===========================================
- Hits         16870    16755     -115     
- Misses        6456     6573     +117     

Impacted Files	Coverage Δ
sleap/skeleton.py	91.15% <ø> (ø)
sleap/nn/training.py	75.39% <50.00%> (-0.80%)	⬇️
sleap/nn/evals.py	55.65% <0.00%> (-34.79%)	⬇️
sleap/nn/callbacks.py	50.89% <0.00%> (-26.79%)	⬇️

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[roomrys]

Note that the decrease in coverage is because we no longer need to actually run training to test whether the Trainer is created as expected through the CLI. The increased coverage (but also testing slow-down) was added in #1130. This PR removes that extra coverage which we weren't even trying to test/verify in the first place.