Update Op.grad, Op.L_op, and aesara.gradient docstrings and type hints

[brandonwillard]

This PR adds a much more informative set of docstrings for Op.grad and Op.L_op. The updated docstrings are clearer about the method inputs and outputs, as well as their underlying mathematical interpretations and relationships.

There is also a bit of refactoring of aesara.gradient;the kind that clarifies function interfaces and helps with typing.

[codecov]

Codecov Report

Merging #1038 (62f50bb) into main (c8eea20) will increase coverage by 0.00%.
The diff coverage is 91.46%.

@@           Coverage Diff           @@
##             main    #1038   +/-   ##
=======================================
  Coverage   79.27%   79.27%           
=======================================
  Files         152      152           
  Lines       47976    47965   -11     
  Branches    10921    10923    +2     
=======================================
- Hits        38031    38023    -8     
  Misses       7438     7438           
+ Partials     2507     2504    -3     

Impacted Files	Coverage Δ
aesara/graph/op.py	86.59% <ø> (ø)
aesara/gradient.py	77.32% <91.46%> (+0.06%)	⬆️