This PR adds rules F821-823:

• F821: undefined name not allowed (very useful for catching bugs, see for instance the missing warnings import)
• F822: everything in __all__ should exist. Critical, but also seems a bit too evident, might not be worth it to have it as a rule? Fine with removing it.
• F823: don't allow "variable used before assignment" --> also good for catching bugs.

I think adding these rules makes sense and does not significantly raise the bar for contributions, while still being good for preventing bugs, which is the philosophy agreed upon in #539. In recent months, a few bugs in main were already identified manually and fixed, which would have been caught by these rules too, e.g. part of the small fixes in #981 , #955, so these rules do correspond with real bugs which occur during development, not some hypothetical scenarios (note also the change in optim_iterator fixes a live bug (albeit small).

Checks to be done before submitting your PR

• python3 -m pytest deepinv/tests runs successfully.
• black . runs successfully.
• make html runs successfully (in the docs/ directory).
• Updated docstrings related to the changes (as applicable).
• Added an entry to the changelog.rst.