Quantization: preserving pre and post forward hooks (#37233)
Summary:
1. While do convert() preserve module's **pre and post forward** hooks
2. While do fusion preserve only module's **pre forward** hooks (because after fusion output no longer the same)
Pull Request resolved: https://github.com/pytorch/pytorch/pull/37233
Differential Revision: D22425141
Pulled By: jerryzh168
fbshipit-source-id: e69b81821d507dcd110d2ff3594ba94b9593c8da
