[Quant] make x86 the default quantization backend (qengine) (#91235)
**Summary**
Make x86 the default quantization backend (qengine) for X86 CPU platforms.
X86 is a unified quantization backend combining goodness of fbgemm and onednn. For more details please see https://github.com/pytorch/pytorch/issues/83888
**Test plan**
python test/test_quantization.py
Pull Request resolved: https://github.com/pytorch/pytorch/pull/91235
Approved by: https://github.com/jgong5, https://github.com/XiaobingSuper, https://github.com/malfet
