Remove Context dependency from Tensor class (#14269)

Remove Context dependency from Tensor class (#14269)

Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/14269

Removes reference to Context proper and instead adds a bool argument for async copy (the same as `copy_`)

For CopyFrom - I haven't tweaked all callsites yet. Instead I rely on a terrible hack that pointer to context is implicitly converted to bool when passed, haha :) It's not a good code and I propose to fix it in a follow up diff (maybe using clangr tooling).

Reviewed By: ezyang

Differential Revision: D13117981

fbshipit-source-id: 7cb1dc2ba6a4c50ac26614f45ab8318ea96e3138