Remove Backend -> THPLayout mapping. (#37527)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/37527
This is yet another place that needs to be updated for adding a new "Backend" and is unnecessary. Instead, just use layout_from_backend and have a map from Layout -> THPLayout.
Other changes:
- rename torch::getDtype and torch::getLayout to torch::getTHPDtype and torch::getTHPLayout since e.g. for layout you are both passing in and returning a "layout" type.
- add NumOptions to Layout to match the dtype/ScalarType formulation.
Test Plan: Imported from OSS
Differential Revision: D21309836
Pulled By: gchanan
fbshipit-source-id: ede0e4f3bf7ff2cd04a9b17df020f0d4fd654ba3
