Fix warning caused by to_int32

While running tf.keras I noticed the following warning:
```
WARNING:tensorflow:From /usr/local/lib/python2.7/dist-packages/tensorflow/python/ops/math_ops.py:3064: to_int32 (from tensorflow.python.ops.math_ops) is deprecated and will be removed in a future version.
Instructions for updating:
Use tf.cast instead.
```

This fix fixes the warning caused by deprecated to_int32.

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