Kernel gets Stack* instead of ArrayRef<IValue> (#16282)

Kernel gets Stack* instead of ArrayRef<IValue> (#16282)

Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/16282

This changes the core kernel abstraction to be a function taking a stack, popping its arguments from the stack and pushing results to the stack,
instead of getting arguments as ArrayRef<IValue> and returning an output IValue.

Caffe2 operators need to have a way to pass in preallocated output tensors.
The convention for them is to get all inputs *and* outputs on the stack and also return all of them, i.e. a caffe2 op will always have inputs == outputs.
This will probably change in later diffs towards making the outputs in-arguments optional in the JIT schema.

Reviewed By: ezyang

Differential Revision: D13792335

fbshipit-source-id: e9cc2b5e438cc4653e1f701633a154b92b604932