[Quant][fx] Remove `remove_quant_dequant_pairs` and fix tests (#84203)
Summary:
- `remove_quant_dequant_pairs` removes ops when a `quant` is followed by a `dequant`
- It looks like the quantized implementation of `layer_norm` only supports float weights, so updated the default qconfig to avoid quantizing the weight param.
- Fixes broken test, `test_norm_weight_bias`. This was the only test that broke, because the default qconfig dict we pass in quantizes the weight. I just pulled the native qconfig object and converted it to a dict.
- Adds in qconfig and backend config support for layernorm
Test Plan:
```
python test/test_quantization.py TestQuantizeFx
python test/test_quantization.py TestQuantizeFxOps
python test/test_quantization.py TestQuantizeFxModels
```
Reviewers:
Subscribers:
Tasks: Fixes https://github.com/pytorch/pytorch/issues/83110
Tags: quant, fx
Differential Revision: [D39395141](https://our.internmc.facebook.com/intern/diff/D39395141)
Pull Request resolved: https://github.com/pytorch/pytorch/pull/84203
Approved by: https://github.com/jerryzh168
