Smooth over some pybind11 integration issues / document common patterns

[ezyang]

Code that uses pybind11 often has to interoperate with legacy code that uses THPObjectPtr. Thus, there are a number of common things that one might have to do, e.g., interconvert between PyObject*, THPObjectPtr, py::handle and py::object. We should document pave the cowpaths, and eventually get rid of THPObjectPtr entirely (if possible.)

[apaszke]

Let's not go too far too quickly. We still haven't fully evaluated pybind. While the quality of bindings doesn't really matter for the JIT, because it's not a hot spot, we need to maintain the lowest possible overhead for tensor/autograd bindings.

[ezyang]

Here's a start: https://fb.quip.com/bX0RA1N2Xng2