fx quant: fix fq when input is quantized and node does not need fq (#49382)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/49382
Fixes an edge case. If the input to the graph is quantized and the
first node does not need activation observation, makes sure that
the observer is not inserted.
Test Plan:
```
python test/test_quantization.py TestQuantizeFxOps.test_int8_input_no_unnecessary_fq
```
Imported from OSS
Reviewed By: jerryzh168
Differential Revision: D25551041
fbshipit-source-id: a6cba235c63ca7f6856e4128af7c1dc7fa0085ea
