Add undefined tensor gradient support to all backward functions (#39400)
Summary:
Adds the ability for all backward functions to accept undefined output gradient arguments. An undefined gradient is a Tensor that was created by the argumentless constructor `at::Tensor()`, where `tensor.defined() == false`.
Also adds new autograd nodes, UndefinedGrad and UndefinedGradBackward, that can be used from within Python code to inject undefined gradients into a backward function. A new test case is added to the backward function unit tests to use the UndefinedGrad node to ensure that undefined gradients do not break any backward functions.
Closes https://github.com/pytorch/pytorch/issues/33138
Pull Request resolved: https://github.com/pytorch/pytorch/pull/39400
Differential Revision: D21936588
Pulled By: albanD
fbshipit-source-id: eccc5f55c77babe6dadcea4249d0c68a3c64e85d
