Update dtgen to add docstring support

[lockshaw]

Description of changes:

dtgen now supports specifying Doxygen docstrings in .struct.toml (top-level docstring example, field docstring example), .enum.toml (top-level docstring example, value docstring example), and .variant.toml (top-level docstring example, value docstring example) files.

Related Issues:

Linked Issues:

• Issue #

Issues closed by this PR:

• Closes #

Before merging:

• Did you update the flexflow-third-party repo, if modifying any of the Cmake files, the build configs, or the submodules?

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This change is 

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 60.47%. Comparing base (d4a92da) to head (3a29c9e).
Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1581   +/-   ##
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  Coverage   60.47%   60.47%           
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  Files         606      606           
  Lines       14723    14723           
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  Hits         8903     8903           
  Misses       5820     5820           

Flag	Coverage Δ
unittests	60.47% <ø> (ø)

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