[Quant][fx] Rename convert_to_reference to convert_to_reference_fx (#81326)
Summary: This commit renames the convert_to_reference function to
convert_to_reference_fx, which is more descriptive and matches
prepare_fx and prepare_qat_fx better.
Test Plan:
python test/test_quantization.py TestQuantizeFx
Reviewers: jerryzh168
Subscribers: jerryh168
Differential Revision: [D37787876](https://our.internmc.facebook.com/intern/diff/D37787876)
Pull Request resolved: https://github.com/pytorch/pytorch/pull/81326
Approved by: https://github.com/jerryzh168