Reduce the output overhead of OutputColumnMaxHistogramObserver by enabling changing bin_nums, Update the observer_test.py
Summary: Current OutputColumnMaxHistogramObserver will output 2048 bins for each column. The file will be extremely large and the dumping time is quite long. However, we only use the min and max finally. This diff enables changing bin_nums by adding an argument. And the default value is set to 16 to reduce dumping overhead. When we need more bins to analyze the results, we only need to change this argument
Test Plan:
buck run caffe2/caffe2/quantization/server:observer_test
{F263843430}
Reviewed By: hx89
Differential Revision: D22918202
fbshipit-source-id: bda34449355b269b24c55802012450ebaa4d280c
