[te] Add BitCast to the IR
Summary: Adds BitCasting to NNC. This will enable fast approximation algorithms implemented directly in TensorExpressions
Test Plan: buck test mode/no-gpu //caffe2/test/cpp/tensorexpr:tensorexpr
Reviewed By: bertmaher
Differential Revision: D25441716
fbshipit-source-id: c97b871697bc5931d09cda4a9cb0a81bb420f4e2
