Refactor MinMax observer (#23902)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/23902
Copied from Daya's diff in pytorch/pytorch #23191
Refactor MinMax observer and create the base observer class to prepare for future observers such as histogram observer.
ghstack-source-id: 89146014
Test Plan:
Added a test test_minmax_observer
buck test mode/dev caffe2/test:quantization -- 'test_minmax_observer'
```
Running 1 tests
Started new test run: https://our.intern.facebook.com/intern/testinfra/testrun/2533274797931635
✓ caffe2/test:quantization - test_minmax_observer (test_quantization.ObserverTest) 0.055 1/1 (passed)
Finished test run: https://our.intern.facebook.com/intern/testinfra/testrun/2533274797931635
Summary (total time 4.26s):
PASS: 1
FAIL: 0
SKIP: 0
FATAL: 0
TIMEOUT: 0
OMIT: 0
```
buck test mode/dev caffe2/test:quantization -- 'test_observer_scriptable'
```
Running 1 tests
Started new test run: https://our.intern.facebook.com/intern/testinfra/testrun/5348024563344195
✓ caffe2/test:quantization - test_observer_scriptable (test_quantization.ObserverTest) 1.762 1/1 (passed)
Finished test run: https://our.intern.facebook.com/intern/testinfra/testrun/5348024563344195
Summary (total time 6.02s):
PASS: 1
FAIL: 0
SKIP: 0
FATAL: 0
TIMEOUT: 0
OMIT: 0
```
Differential Revision: D16663221
fbshipit-source-id: 3d0e1aa9e4d27808e61b10604782606de067a34a
