Allow dispatch based on tensor list args (#17522)

Allow dispatch based on tensor list args (#17522)

Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/17522

Dispatch is still based on the first tensor arg, but that first "tensor arg" is now allowed to be a tensor list.
That is, the first argument that is either Tensor or TensorList will be the deciding factor for dispatch.
If it is a TensorList, then that TensorList must not be empty or dispatch will fail.

Reviewed By: ezyang

Differential Revision: D14235840

fbshipit-source-id: 266c18912d56ce77aa84306c5605c4191f3d882b