Fix bound partial for python 3.10

[fehiepsi]

Fixes #3095 and resolves #3018.

In Python <= 3.9, _bound_partial(_bound_partial(f)).func != f but for some reasons, they are the same in Python 3.10. This PR tries to address such issue by registering func as a slot of _bound_partial.

[fehiepsi]

For some reason, the change is failing for infer_discrete with temperature=1 and funsor backend. I have been debugging this but still haven't figured it out. Edit: In ReentrantMessenger, instead of returning _bound_partial(fn), we return wraps(fn)(_bound_partial(fn)) hence _bound_partial(_bound_partial(fn)).func is fn, which faces the same issue as the one we wanted to address in this PR.

[fritzo]

LGTM Valiant effort solving this @fehiepsi!

One thing to keep in mind is that classes with __slots__ might not be pickleable (they weren't in Python 2, and I'm not sure if that's been fixed in Python 3). Let's keep an eye out for new pickling errors this might introduce.

[Balandat]

Are there plans to put out a new release with these changes anytime soon? Looks like the latest release 1.8.1. still doesn't support python 3.10.

[fritzo]

Hi @Balandat sure we can push out a release this week. Thanks for the ping

[fritzo]

@Balandat just released Pyro 1.8.2.

[Balandat]

Woohoo, thanks! Great customer service! Where do I leave my yelp review?