feat(useless-rewrite): Useless rewrite from `log1mexp(log1mexp(x))` to `x`

[lmmx]

Motivation for these changes

Picking up this ticket on the PyData Global 2023 OSS sprint :runner:

• #471

The motivating issue concerned an issue using PyMC's Censored functionality which was traced back to the need for a "useless rewrite":

• replacing a $log(1 - exp(x))$ within another $log(1 - exp(x))$
  • i.e. a $log(1 - exp(log(1 - exp(x))))$
• by just an $x$.

Implementation details

1. :broom: refactor(nan switch): DRY out nan switch rewrite function, easier to follow important parts :broom:

• The first change is in preparation for adding a new case, which is to simplify the case handling (avoid repetition).
• This is achieved by putting an "inner function" (function within a function) that captures the x and node from the function body in its scope, meaning we don't need to pass them as parameters, so the body of each case becomes simpler
• Every case has a "nan switch", so this trick lets us avoid repeating ourselves but retaining clarity about the variables we're using.

2. :writing_hand: Add new useless rewrite case :writing_hand:

• The previous operation and the node operation are both going to be log1mexp for this case
• The condition for the case is that x >= 0 (confirm?)

Checklist

• [x] Explain motivation and implementation 👆
• [x] Make sure that the pre-commit linting/style checks pass.
• [x] Link relevant issues, preferably in nice commit messages.
• [x] The commits correspond to relevant logical changes. Note that if they don't, we will rewrite/rebase/squash the git history before merging.
• [ ] Are the changes covered by tests and docstrings?
• [ ] Fill out the short summary sections 👇

Major / Breaking Changes

• N/A

New features

• "Useless rewrite" to optimise $log(1 - exp(log(1 - exp(x))))$ into just $x$

Bugfixes

• Would resolve #471

Documentation

• ...

Maintenance

• Took an opportunity to tidy up the 'nan switch' cases into a dict, which is clearer to read than the repetitive if blocks

[lmmx]

I think to finish this I need to add a test here

https://github.com/pymc-devs/pytensor/blob/c10c376b173402eb06bed9aff56d0c77fd21dd79/tests/tensor/rewriting/test_math.py#L1945-L1971

[ricardoV94]

I think to finish this I need to add a test here

https://github.com/pymc-devs/pytensor/blob/c10c376b173402eb06bed9aff56d0c77fd21dd79/tests/tensor/rewriting/test_math.py#L1945-L1971

Sounds about right

[ricardoV94]

The condition for the case is that x >= 0 (confirm?)

The valid condition is that x <= 0 for which the inner log1mexp is defined. x > 0 shoud yield nan, as that would lead to taking the log of a negative number.