Record in Declarations.yaml which return tensors are differentiable

[ezyang]

max_pool2d returns an output tensor and an indices tensor; only the output tensor is differentiable. prelu_backward, on the other hand, returns a grad_input and grad_weight; both are differentiable.

We need to know which outputs are differentiable, because those are the only ones we should set_flags on. However, at the moment, Declarations.yaml does not record this information. In fact, I am not sure we have this information at all, except perhaps implicitly, in the sense that no non-NN ATen method returns two differentiable outputs (this is hardcoded into gen_variable_type.py). I'm not exactly sure how we should add the information to the source cwrap/yaml files.

Edit. It might be OK to just assume that Tensor is differentiable but IndexTensor is not.