Better name for `ParallelModuleBase`

[duboism]

Hello,

We (@mpigou and I) think that the class ParallelModuleBase should be called differently to better reflect its functionality. The current name comes from FEW where the notion of module meant the different steps in the computation.

We propose BackendConsumer (the more precise technically) or BackendDispatcher (which is not wrong and maybe a bit simpler to understand).

[mikekatz04]

This is a fair point. I will update this in due time since it is attached to all the other codes.