Change exception to warning (#25408)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/25408
Change exception to warning so that observer can be called with no data and still provide a scale and zero-point.
ghstack-source-id: 89267768
Test Plan:
buck test mode/dev caffe2/test:quantization -- 'test_minmax_observer'
buck test mode/dev caffe2/test:quantization -- 'test_observer_scriptable'
Differential Revision: D17116524
fbshipit-source-id: db4d76e882b57f23161dced846df3a0760194a41
