Add function to load metrics

[gitttt-1234]

This PR adds a new function: load_metrics() to load metrics from a model checkpoint directory or a metrics.npz file and returns a dictionary with all the saved metrics

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.50%. Comparing base (ff91433) to head (328e40a).
⚠️ Report is 40 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #334      +/-   ##
==========================================
- Coverage   95.28%   93.50%   -1.79%     
==========================================
  Files          49       49              
  Lines        6765     6971     +206     
==========================================
+ Hits         6446     6518      +72     
- Misses        319      453     +134     

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