Document aihwkit.simulator.rpu_base in readthedocs

[diego-plan9]

Description and motivation

Currently, aihwkit.simulator.rpu_base is not properly displayed in the API reference. This is due to not being able to compile the simulator in the readthedocs build environment, as it does not allow the same flexibility when installing external packages (and at the moment the bindings need to be compiled in order to produce its documentation correctly).

It would be great to have online documentation for that module as well, in the same way as the rest of the modules.

Proposed solution

Alternatives and other information

We might have some luck using .pyi stubs - even if they seem not fully supported yet in sphinx (https://github.com/sphinx-doc/sphinx/pull/4824), there are pointers for other options such as https://github.com/readthedocs/sphinx-autoapi.