Fix warning in losses_utils.py

While debugging 24577 I noticed the following warning:
```
WARNING:tensorflow:From /usr/local/lib/python3.6/dist-packages/tensorflow/python/keras/utils/losses_utils.py:170: to_float (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.

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