* Fix gc_refs assertion failure
Ensure that each THPVariable -> THPFunction reference contributes one
ref count to the THPFunction by creating a new shared_ptr for each ref.
Because multiple shared_ptrs can again manage a single THPFunction, it's
not safe to use std::weak_ptr where it may point to a PyFunction. It's
still safe to use weak_ptr for grad_accumulator since these are never
PyFunctions.
Fixes#1626
* Remove stale comment
Because of this Variables can no longer appear in the graph.
Every usage of a leaf Variable will leave an AccumulateGrad
function that has no outputs, but modifies var.grad as a side
effect.
The core autograd Variable, Function, and Engine no longer depend on the
Python API. This let's us implement functions in C++. In the future, we
can also multithread engine and release the GIL for most of the
non-Python backwards.
