Add helper function to find all inputs needed to a compile a graph

[jessegrabowski]

Description

It'd be nice to have a tool to automatically make the required inputs to a function. The function already exists in PyMC as pymc.pytensorf.inputvars, but I think this should be in pytensor itself.

There's also pytensor.graph.basic.graph_inputs, but this returns constants and shared variables. Given an arbitrary graph z, I'd like to be able to do f = pytensor.function(get_input_vars(z), z).

[ricardoV94]

As suggested let's call it required_graph_inputs to distinguish from the existing one