Update mlir.py
Please don't use "add file"/"update file"/"fix file"/etc. commit messages. These are hard to reason about when looking at the history of the file/repository. Instead, please write explanatory git commit messages.

The commit message is also the title of the PR if the PR has only one commit. It is thus twice important to have commit messages that are relevant, as PRs would be easier to understand and easier to analyze in search results.

For how to write good quality git commit messages, please consult https://cbea.ms/git-commit/ 
Break up the code into smaller functions.

Split the convert_graph_def() function into functions for importing the GraphDef, running the pass pipeline, and converting the MLIR module to text. Add unit tests.

Add unit tests for the convert_graph_def() function to ensure that it is working as expected. Add docstrings to the functions.

Add docstrings to the convert_graph_def(), _import_graph_def(), _run_pass_pipeline(), and _convert_mlir_module_to_text() functions to explain what they do and how to use them. Improve the type hints.

Update the type hint for the convert_graph_def() function to make it more informative.