Two bug fixes: mix checkpointing and dtype

[danielkorzekwa]

What does this PR do?

Two bug fixes:

1. Saving mip checkpoint (correctly saving model code files)
2. Passing dtype as object instead of string to calculate_losses_pipeline and load_and_shard_model

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 74.38%. Comparing base (1d8bd20) to head (c162da2).
⚠️ Report is 3 commits behind head on feature/compress.

Additional details and impacted files

@@                 Coverage Diff                  @@
##           feature/compress     #718      +/-   ##
====================================================
- Coverage             74.47%   74.38%   -0.09%     
====================================================
  Files                   183      185       +2     
  Lines                 18416    18521     +105     
====================================================
+ Hits                  13715    13777      +62     
- Misses                 4701     4744      +43     

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