Back out "[pytorch][PR] [JIT] Add `__prepare_scriptable__` duck typing to allow replacing nn.modules with scriptable preparations"
Summary: Original commit changeset: 4ddff2d35312
Test Plan: sandcastle
Reviewed By: zhangguanheng66
Differential Revision: D25061862
fbshipit-source-id: 1d0cc5a34b8131ac88304f24394b677131d28e39
