Support observer without any data calibration (#24923)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/24923
Replace exception with warning for un initialized min/max values to support creation of quantized models without observers.
ghstack-source-id: 89003800
Test Plan: Replace error message with warning for observers
Differential Revision: D16923660
fbshipit-source-id: 9927ed4e4ee977c1388595ddef042204f71076a4
