Use C10Tensor in the dispatcher (#15195)

Use C10Tensor in the dispatcher (#15195)

Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/15195

This removes the use of caffe2::Tensor or at::Tensor in the c10 dispatcher and only uses C10::Tensor.
It also changes output tensors to be passed as `const Tensor&` instead of `Tensor*` because we otherwise can't forward them in operator_c10wrapper.h.

Reviewed By: ezyang

Differential Revision: D13461640

fbshipit-source-id: 7f79925a7d60f01660a24bbfda47391af0c70ed3