[Quant][fx][bc-breaking] Remove backend_config_utils.py (#89810)
Summary: Previously under torch/ao/quantization we have
backend_config/utils.py and fx/backend_config_utils.py, which
was confusing. This commit deletes the latter and moves
everything there to more suitable util files.
BC-breaking note: The following public APIs under the
`torch.ao.quantization.fx.backend_config_utils` namespace
are removed in this commit.
```
get_quantize_handler_cls
get_fusion_pattern_to_fuse_handler_cls
get_native_quant_patterns
get_pattern_to_quantize_handlers
```
Test Plan:
python test/test_quantization.py TestQuantizeFx
python test/test_quantization.py TestQuantizeFxOps
Reviewers: jerryzh168, vkuzo
Subscribers: jerryzh168, vkuzo
Pull Request resolved: https://github.com/pytorch/pytorch/pull/89810
Approved by: https://github.com/jerryzh168
