fx quant: make sure observer is inserted before a quantized output (#49420)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/49420
Before: if an output was marked as quantized, it could actually not
be quantized, if the previous node was not quantized.
After: if an output was marked as quantized, it will be quantized
regardless of the quantization status of the previous node.
Test Plan:
```
python test/test_quantization.py TestQuantizeFxOps.test_quant_output_always_observed
```
Imported from OSS
Reviewed By: jerryzh168
Differential Revision: D25566834
fbshipit-source-id: 84755a1605fd3847edd03a7887ab9f635498c05c
