Fix WARNING in BatchNormalization (#18315)

The keep_dims for reduce_mean has been deprecated and replaced
with keepdims. This casues the following WARNING in BatchNormalization:
```
normalization.py:584: calling reduce_mean (from tensorflow.python.ops.math_ops) with keep_dims is deprecated and will be removed in a future version.
Instructions for updating:
keep_dims is deprecated, use keepdims instead
```

This fix fixes the warning in BatchNormalization.

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