fx quant: move {input|output}_quantized_idxs cfg from convert to prepare (#49238)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/49238
Moves the `input_quantized_idxs` and `output_quantized_idxs` options
from the convert config to the prepare config. This is done because
these operations are related to placing observers, which is numerics
changing during QAT.
The next PR will adjust the behavior of `input_quantized_idxs` in
prepare in QAT to prevent placing a fake_quant at the input if the
input is marked quantized. Placing a fake_quant there can lead to
numerical inaccuracies during calibration, as it would start with
scale=1 and zp=0, which may be different from the quantization
parameters of the incoming quantized input.
Test Plan:
```
python test/test_quantization.py TestQuantizeFx
```
Imported from OSS
Reviewed By: jerryzh168
Differential Revision: D25498762
fbshipit-source-id: 17ace8f803542155652b310e5539e1882ebaadc6
