Fix/style fixes

[kwikwag]

1. Content and background

Some coding style suggestions (including one possible bugfix).

2. Summary of corrections

I went over the following:

1. Removed Optional where it was incorrectly specified in a type annotation Added some Literal type annotations.
   Optional should only be specified when the argument can receive None as a value, which was not true in most cases. From Python docs:

   Optional[X] is equivalent to X | None (or Union[X, None]).

2. Moved traceback printing calls to logging.

3. Raise exceptions instead of sys.exit(). This makes it easier to use within another script.

4. Do not use base except. This has the side-effect of catching KeyboardInterrupt too which is most likely not what is wanted.

5. Ran black for styling and went over some of flake8 linting comments (e.g. remove f prefix when the f-string has no placeholder, etc.).

The latter revealed that a call to gs.export_onnx() was using an undefined variable graph. This bug was introduced in commit 85147eb,

I had a mistake in one of the commits with a call to warn_tb that I didn't correct until a later commit. But the final results is OK.

If need be, I can prepare a pull request with only a subset of these changes. I avoided making any functional changes, only focusing on style.

[PINTO0309]

I don't have any objections to the proposed changes themselves, but there are too many changes to review. Can you split up the pull requests into separate requests for each requirement? The scope of the impact is too large.