Set seed by default when entering pyro_backend

[neerajprad]

Requires:

• fixes to Pyro Exclude seed handler from the PYRO_STACK pyro#2061
• fixes to NumPyro Make behavior of nested seed handler consistent with Pyro numpyro#397
• fixes to funsor Add seed handler and tests funsor#269

[neerajprad]

I think this should be good to merge once we start using more recent commits from numpyro and funsor. Should we start using the master branches of these and dev branch of Pyro for testing?

[fritzo]

Yes, let's do that. I'll update this PR.