dbr quant function fusion [2/x]: use fusion for observation and inference (#71781)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/71781
The previous PR added information about fusions found in the subgraphs.
This PR uses that information for:
1. inserting observers at the end of fusions and not in the middle
2. during inference, replacing the original op with the fused op. The
way this is implemented is that the base op is replaced with the fused op,
and all other ops are replaced with identity functions.
Test Plan:
```
python test/test_quantization.py TestQuantizeDBR.test_fusion_functions
```
Reviewed By: jerryzh168
Differential Revision: D33775097
Pulled By: vkuzo
fbshipit-source-id: 12249b85b2f7ba7545a54872aeb5f1ff2fc928cf
(cherry picked from commit 0db4324ea9250efc37748e6034b926acffebbad5)
