Fix #13731 by adding HistogramdFixedWidth in hidden_ops.txt and create the python wrapper (#13781)

* Fix 13731 by adding HistogramdFixedWidth in hidden_ops.txt and create the python wrapper

so that both api compatibility and test utility code in contrib could be
preserved. See https://github.com/tensorflow/tensorflow/pull/13731#issuecomment-337186002


for reference.

Signed-off-by: Yong Tang <yong.tang.github@outlook.com>

* Add underscore (`_histogram_fixed_width`) in calling gen_math_ops.py

Signed-off-by: Yong Tang <yong.tang.github@outlook.com>

* clang-format -i histogram_op.cc

Signed-off-by: Yong Tang <yong.tang.github@outlook.com>