[quant][pt2e] Fix handling for SharedQuantizationSpec (#106922)
Summary:
Previously if we have:
```
conv1 -> cat
conv2 /
```
and configure output of conv1/conv2 to be int8 quantized, and cat also int8 quantized and with shared inputs,
it will not produce expected results (input of cat will not be shared)
The problem is that there is some missing checks when inserting observers for input for cat
This PR fixes the problem.
Fixes: https://github.com/pytorch/pytorch/issues/106760
Test Plan:
python tes/test_quantization.py TestQuantzePT2E.test_shared_qspec
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Subscribers:
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Pull Request resolved: https://github.com/pytorch/pytorch/pull/106922
Approved by: https://github.com/kimishpatel
