[docstring] Fix docstrings for `CodeGen` (#26821)
* remove docstrings CodeGen from objects_to_ignore
* autofix codegen docstrings
* fill in the missing types and docstrings
* fixup
* change descriptions to be in a separate line
* apply docstring suggestions from code review
Co-authored-by: Yih-Dar <2521628+ydshieh@users.noreply.github.com>
* update n_ctx description in CodeGenConfig
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Co-authored-by: Yih-Dar <2521628+ydshieh@users.noreply.github.com>
