[quant] Input-weight equalization - branch support (#62366)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/62366
In the case of models with branches, we are unable to equalize the branching part in the graph.
For example, given this graph:
```
conv2
/ \
x -> conv1 -> add
```
After prepare, we will ignore the branched layers (conv1 and conv2) and will not insert the equalization observers. A warning message will also be printed with the layers that are unable to be equalized.
```
conv2 -> out_quant_obs2
/ \
x -> input_quant_obs -> conv1 -> out_quant_obs1 -> add
```
Test Plan:
`python test/test_quantization.py TestEqualizeFx.test_input_weight_equalization_prepare`
Imported from OSS
Reviewed By: malfet, supriyar
Differential Revision: D29982585
fbshipit-source-id: 706297e7f1861975998dfa83e7ca59af09d80618
