Fix a bug in quantize_linear (#20711)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/20711
For uint8_t, ```std::numeric_limits::digits``` returns 8;
For int8_t, ```std::numeric_limits::digits``` returns 7.
FBGEMM wants to get the ```qparams.precision``` to be always 8 for both int8_t and uint8_t.
Reviewed By: jerryzh168
Differential Revision: D15410695
fbshipit-source-id: 17dc3842d7c426947454c201bcb167b87b7301ce
