[LT] Upstream backend interfaces (#67927)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/67927
BackendData - represents 'tensor data' in opaque backend storage
LoweringContext - interface for performing backend-specific IR lowering
BackendImplInterface - interface for lazy tensors backends to implement
Reorgs backend-related files into lazy/backend subdir
includes a few small fixes, which were made on lazy_tensor_staging but need to be back-ported to master.
Test Plan: used by lazy_tensor_staging branch
Reviewed By: desertfire
Differential Revision: D32142032
fbshipit-source-id: 828c717bcd0d511876e64ad209b50f7bfb10cec5
