message: 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.
diff: @@ -167,8 +167,8 @@ def compute_weighted_loss(losses,
  losses, None, sample_weight)
  losses = ops.convert_to_tensor(losses)
  input_dtype = losses.dtype
  - losses = math_ops.to_float(losses)
  - sample_weight = math_ops.to_float(sample_weight)
  + losses = math_ops.cast(losses, dtypes.float32)
  + sample_weight = math_ops.cast(sample_weight, dtypes.float32)
  try:
  # Broadcast weights if possible.
  