[Quant] Make x86 backend default when querying qconfig (#85461)
This PR is a follow-up of #84329 [[Quant] Add unified x86 quant backend](https://github.com/pytorch/pytorch/pull/84329)
It makes `x86` backend default when querying `qconfig`. Users get x86's qconfig/qconfig_mappings if backend is not specified.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/85461
Approved by: https://github.com/jgong5, https://github.com/vkuzo
