default options for OutputTensorCopyFrom (#15248)

default options for OutputTensorCopyFrom (#15248)

Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/15248

OutputTensorCopyFrom takes four arguments: index, a source Tensor, TensorOptions and whether we want to perform an async call.
We want to provide some default option for TensorOptions, (1). default device to context_.device() (2). default dtype to input.dtype(). User can also explicitly provide these options to override default values.

next diff will change the order of TensorOptions parameter so that user don't need to write down tensor options unless they want to override.

Reviewed By: dzhulgakov

Differential Revision: D13453824

fbshipit-source-id: 87401f81c7c3f9fd3d8936c710e6c2e04a59b689