[quant] Return default qconfig when backend is 'none' (#38407)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/38407
We can still run some quantized tests even when fbgemm/qnnpack isn't enabled
Test Plan:
python test/test_quantization.py
Imported from OSS
Differential Revision: D21554257
fbshipit-source-id: e4fa8f61f6a6717881c00620ed7938c01ffbf958
