Upgrade to PyTorch 1.8

[fritzo]

This documents our upgrade plan to PyTorch 1.8, which breaks pyro.distributions as in Pyro 1.5.1 release. PyTorch 1.8.0 release is scheduled for sometime in March 2021.

Plan

• Bump to version 1.5.2 #2755 Release Pyro 1.5.2 which constrains to torch<=1.7.2.
• Upgrade to PyTorch 1.7 #2757 Upgrade dev branch to PyTorch 1.7.0, fixing tests.
• Update dev branch to use PyTorch 1.8 prerelease #2753 Pin Pyro dev branch to PyTorch nightly build, and fix many breaking changes.
• Add a Distribution.infer_shapes() method to statically infer shapes #2739 Implement Distribution.infer_shapes().
• ...
• After PyTorch 1.8 release, pin Pyro dev back to PyTorch release and release Pyro 1.6.

cc @neerajprad