Replace math_ops.div with math_ops.divide

While testing the issue of 23709, noticed several warnings:
```
W0217 12:33:32.155959 139635342989120 deprecation.py:323] From /usr/local/lib/python3.6/dist-packages/tensorflow/python/ops/math_grad.py:106: div (from tensorflow.python.ops.math_ops) is deprecated and will be removed in a future version.
```
This fix replaces math_ops.div with math_ops.divide to remove the warning

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