* Fix segfault in autograd:
1) Every "output" variable must have a grad_fn or grad_accumulator
2) compute_partial_exec_callbacks uses Python errors
* assertRaisesRegexp was renamed assertRaisesRegex in 3.2
* Use HANDLE_TH_ERRORS macro
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.
