Add `tf.unravel_index` as an equivalent of `np.unravel_index`

This fix tries to address the issue raised in 2075 where
there was no implementation of  `tf.unravel_index`.
The `tf.unravel_index` could be quite useful in many places.
This fix adds the `tf.unravel_index` in CPU kernel. Note `order`
in `np.unravel_index` has not been added yet.

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