[fx] allow custom behavior for args, kwargs, and bool (#45193)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/45193
This change makes it possible to subclass the tracer to add additional
behavior when you know something about the shape of the Proxy objects,
by overriding the defaults for how the tracer tries to make something iterable,
looks for keys for **kwargs, or tries to convert to a boolean.
An example test shows how this can be used to tag inputs with shapes.
It can also be used combined with create_node to do type propagation during
tracing to fullfil requests like iter.
Test Plan: Imported from OSS
Reviewed By: jamesr66a
Differential Revision: D24258993
Pulled By: zdevito
fbshipit-source-id: 6ece686bec292e51707bbc7860a1003d0c1321e8
