first pass at ruff linter fixes

[sritchie]

This PR:

• fixes many ruff linter errors, removing unused imports and variables
• disables ruff formatting and linting for ipynb files

I verified that all tests pass on a cloud VM.

[georgematheos]

@sritchie thank you for making this PR!

Two questions, from a quick look --

1. Have you run the tests successfully on this? I ask because this removes a few lines of the form from . import X, in Y/__init__.py files, and I'm wondering if this breaks any of the tests which are expecting to be able to import Y.X.
2. It looks like this PR changes a number of lines of the form varname = expression() to expression(), and for most of these, expression() does not have a side effect. Is the best bet to go through and remove all of these manually? (Maybe it would be better to try to automatically delete all these lines, and then hope the tests catch if this has broken anything relying on side effects.)

[sritchie]

@georgematheos it would definitely be good to go remove those lines, I just didn't know enough myself to know which lines were relying on side effects and which were not. I'm happy to go get it done.

Let me try the tests on this today, and if they look good, what do you think about giving an approval here?

[sritchie]

note to stare at this one more closely

[georgematheos]

@nishadgothoskar can you please comment on this? -- Do we need to call _rasterize_bwd_custom_call here?

[georgematheos]

(@sritchie I am not familiar with all the code in renderer.py, so we probably need Nishad's input here.)