col max hist observer
Summary:
Add InputColumnMaxHistogramNetObserver and InputColumnMaxHistogramObserver to dnnlowp observers.
Sample output histogram at /mnt/public/amyyang/test/col_max_test.log (generated for ctr_web_feed)
```
columns:
"op_index",
"input_idx",
"blob_name",
"col_idx",
"min",
"max",
"nbins"
```
Test Plan: Tested with ctr_web_feed
Reviewed By: csummersea
Differential Revision: D18194229
fbshipit-source-id: 1402fcdc174a1f52744c850f5e2cc3bdc73c3a45
