Move TensorImpl::CopyFrom to caffe2::Tensor (1/2) (#14656)

Move TensorImpl::CopyFrom to caffe2::Tensor (1/2) (#14656)

Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/14656

This diff doesn't move it yet, but prepares it to be moved, i.e. removes all access to class internals.

dzhulgakov: Please comment on if you think it still makes sense to land this even though it's not blocking anymore since we're going to move at::CopyBytes anyhow.

ezyang: There's some changes in the implementation, especially handling undefined dest tensors. Please review carefully.

Reviewed By: ezyang

Differential Revision: D13287688

fbshipit-source-id: 17800ca8a79ab1633f23be58d96f99a160d8ed24