Minor bug fixes

[gitttt-1234]

In this PR, we introduce the following changes:

• Update the default value of trainer_config.trainer_devices.

• Fix the default run name logic: it now uses the number of labeled frames (instead of the number of label objects).

[codecov]

Codecov Report

❌ Patch coverage is 80.00000% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 95.35%. Comparing base (ff91433) to head (2f2d3a2).
⚠️ Report is 26 commits behind head on main.

Files with missing lines	Patch %	Lines
sleap_nn/training/model_trainer.py	66.66%	1 Missing ⚠️

❌ Your patch status has failed because the patch coverage (80.00%) is below the target coverage (95.00%). You can increase the patch coverage or adjust the target coverage.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #317      +/-   ##
==========================================
+ Coverage   95.28%   95.35%   +0.07%     
==========================================
  Files          49       49              
  Lines        6765     6958     +193     
==========================================
+ Hits         6446     6635     +189     
- Misses        319      323       +4     

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