[torch/deploy] Minor housekeeping in interpreter_impl
Summary:
1. Delete dead code relating to maskrcnn_benchmark extension module
2. Add some more commentary on why we define a meta path finder
isthisimpact
Test Plan: sandcastle
Reviewed By: wconstab
Differential Revision: D28078211
fbshipit-source-id: cfc6f47861c14ec7482b55ee585504271ae0f365
