Merge TensorImplPtr into TensorPtr.

Summary:
Looks like the users can totally share the same Tensor backed by the same TensorImpl.
That way sharing a Tensor happens naturally, and whoever wants to build a truly separate Tensor would just unsafeGetTensorImpl() and go ahead. Cloning also seems straightforward - just copy the data\metadata from an existing TensorPtr into a new one, like we do now.

Reviewed By: swolchok

Differential Revision: D63557912

fbshipit-source-id: 98939ab410ab4c13ccad1c1d64189ed51fce3621