Add docstring and type hints to DistributionMeta

[canyon289]

Description of your problem

Please add more documentation to DistributionMeta. This code block controls the logic for basically all distributions but its unclear what exactly its doing, or why.

For example whats cls_dict, how is it created, why is it needed?
https://github.com/pymc-devs/pymc/blob/main/pymc/distributions/distribution.py#L76

Note: This not a beginner issue, one of the core devs should do it to ensure its fully descriptive and contains all nuances

[canyon289]

With some googling I'm realizing that clsdict seems to be some convention for ABCMeta functionality. Nonetheless I think we should still document it in PyMC for these following reasons

1. It doesnt seem to be a common pattern so im not finding many examples
2. This portion of the code is loaded with tons of "hardcore" python functionality, from Metaclasses to use of __new__ overwrite, to singledispatch
3. A lot of this then references other libraries like AePPL, in particular using design choices that aren't immediately obvious

The subset of people in the world that have enough stats knowledge to contribute to PyMC, know Python well enough to know what each individual line does, and know the design patterns of PyMC, AePPL, and Aesara is very small. I'm definitely not one of them :)

Documentation will go a long way to help make this critical part of the library less challenging to understand!

https://programtalk.com/python-examples/abc.ABCMeta.__init__/

[ricardoV94]

See #5308

[OriolAbril]

It looks like this was addressed already. Can it be closed?