add checkpoint load, model export, validation mode to training scripts

[alexh-deeplite]

Adding new command-line arguments and functionality to training scripts to support use cases where:

• fine tune a pretrained model that isn't present in the zoo
• export a pretrained model
• validate a pretrained model

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 86.86%. Comparing base (3654f7b) to head (acdcc7f).
Report is 24 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #403      +/-   ##
===========================================
+ Coverage    86.63%   86.86%   +0.23%     
===========================================
  Files           70       70              
  Lines         1474     1500      +26     
===========================================
+ Hits          1277     1303      +26     
  Misses         197      197              

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[alexh-deeplite]

this PR is to be merged by #388