Add customerized kernel implementation for clip_by_value

This fix tries to address the issue raised in 7225 where
`tf.clip_by_value` does not have a custom kernel and reused
`tf.maximum` and `tf.mimimum`. In case scalar values are passed
to `tf.clip_by_value`, unnecessary memory might incur.

This fix adds the customerized kernel implementation for
`tf.clip_by_value`.

This fix fixes 7225.

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