Add a to_pyro_module_() helper

[fritzo]

Addresses #2161

This aims to make it easier to convert 3rd party nn.Module instances to PyroModules, as described in @simeneide's forum post.

The safest way to do this appears to be to update the .__class__ attribute in-place:

module.__class__ = PyroModule[module.__class__]
module._pyro_... = ...  # initialize PyroModule attributes

While this appears intrusive, it is equivalent to replacing nn.Linear -> PyroModule[nn.Linear] and so forth in the 3rd-party code that created the instance. It should therefore also be compatible with serialization.

🐛 Bugfixes

This PR also fixes two bugs surfaced by new tests:

• A bug whereby ._pyro_set_supermodule() was being bypassed due to nn.Module subclasses calling .add_module() rather than .__setitem__() directly.
• A recursion bug in tests.common.assert_close("foo", "foo")

Tested

• added a test that to_pyro_module_(m) is equivalent to recursively inserting PyroModule[-]
• strengthen existing tests