[quant][graphmode][fx] Add reference quantized conv module (#63828)

[quant][graphmode][fx] Add reference quantized conv module (#63828)

Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/63828

Added reference quantized conv module for the custom backend flow, the reference quantized module will
have the following code:
```
        w(float) -- quant - dequant \
        x(float) ------------- F.conv2d ---
```
In the full model, we will see
```
        w(float) -- quant - *dequant \
        x -- quant --- *dequant --  *F.conv2d --- *quant - dequant
```
and the backend should be able to fuse the ops with `*` into a quantized linear

Test Plan:
python test/test_quantization.py TestQuantizeFx.test_conv_linear_reference

Imported from OSS

Reviewed By: vkuzo

Differential Revision: D30504749

fbshipit-source-id: e1d8c43a0e0d6d9ea2375b8ca59a9c0f455514fb