Add type hints / documentation for `mode` argument

[ricardoV94]

Description

As discussed in https://github.com/pymc-devs/pymc/pull/7212#discussion_r1553967791 it's not clear what the signature / type is.

Besides a Mode object there is a set of strings that are allowed: https://github.com/pymc-devs/pytensor/blob/a76172ee6a9753266150fc7bac4dd66906967a26/pytensor/compile/mode.py#L484-L534

[OriolAbril]

I have found:

• https://pytensor.readthedocs.io/en/latest/glossary.html#term-Mode
• https://pytensor.readthedocs.io/en/latest/library/compile/mode.html#pytensor.compile.mode.Mode

maybe its better to link to the first one for docstrings?