[quant][fx][graphmode] Add support for conv add pattern in backend_config_dict (#69778)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/69778
This PR extends fusion pattern support from simple sequence of ops to a simple
subgraph like conv - add
```
x - conv ---\
y ---------add ---- ouptut
```
where input x, y and output are observed/quantized
Test Plan:
```
python test/fx2trt/test_quant_trt.py TestQuantizeFxTRTOps.test_conv_add
```
Imported from OSS
Reviewed By: vkuzo
Differential Revision: D33024528
fbshipit-source-id: 5c770c82c8f693fabdac5c69343942a9dfda84ef
