[quant][api] Add assert for backend in get_default_qconfig related apis (#86259) (#87331)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/86259
Add assertion to make sure backend is one of "fbgemm", "x86", "qnnpack" and "onednn"
for get_default_qconfig, get_default_qat_qconfig, get_default_qconfig_mapping and get_default_qat_qconfig_mapping
Test Plan:
python test/test_quantization.py -k test_get_default_qconfig_mapping
Imported from OSS
Reviewed By: jcaip
Differential Revision: D40236474
Pull Request resolved: https://github.com/pytorch/pytorch/pull/87331
Approved by: https://github.com/andrewor14
