Fix scriptability for Observer (#25197)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/25197
Ensure that observer code remains scriptable after addition of warnings
ghstack-source-id: 89022474
Test Plan: buck test caffe2/test:quantization -- 'test_observer_scriptable \(test_quantization\.ObserverTest\)' --print-passing-details
Differential Revision: D17059486
fbshipit-source-id: 70ea9ee39f0b896c7801e168666f88c156dbf15b
