fx quant: do not insert observers at quantized inputs (#49239)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/49239
Context: the existing implementation of `quantized_input_idxs` is convert-only.
Therefore, observers are inserted between the input and the first
quantized node. This is a problem during QAT, because the initial
input is a fake_quant, and it starts with scale=1 and zp=0. This does
not match the quantization parameters of the graph input, which can
lead to incorrect numerics.
Fix: do not insert observer for a quantized input.
Test Plan:
```
python test/test_quantization.py TestQuantizeFx
```
Imported from OSS
Reviewed By: jerryzh168
Differential Revision: D25499486
fbshipit-source-id: 303b49cc9d95a9fd06fef3b0859c08be34e19d8a
