Add simple save model test

[jeswan]

This PR closes #1222 by adding save model tests for the simple runscript. These tests check the model saving behavior by:

1. Overfitting a model and checking the last model weights are different than the best model weights
2. Checking that only the best model is saved with the do_save_best argument.
3. Checking that only the last model is saved with the do_save_last argument.

This PR also corrects a small bug in saving behavior (required for the tests to pass).

[codecov]

Codecov Report

Merging #1227 (efacf8a) into master (1ab34a4) will decrease coverage by 0.02%.
The diff coverage is 16.66%.

@@            Coverage Diff             @@
##           master    #1227      +/-   ##
==========================================
- Coverage   55.88%   55.86%   -0.03%     
==========================================
  Files         149      149              
  Lines       10819    10825       +6     
==========================================
+ Hits         6046     6047       +1     
- Misses       4773     4778       +5     

Impacted Files	Coverage Δ
jiant/utils/torch_utils.py	51.08% <16.66%> (-2.41%)	⬇️

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[zphang]

Left a comment, but I'm fine merging for now.