Add support of `drop_negatives` for `tf.unsorted_segment_sum` (#13055)

* Add support of `drop_negatives` for `tf.unsorted_segment_sum`

This fix tries to address the issue raised in 478 by adding the
support of `drop_negatives` for `tf.unsorted_segment_sum`
so that it is possible to skip entries (when index = -1)

This fix fixes 478.

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

Add test case for `drop_negatives` of `tf.unsorted_segment_sum`

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

Remove quote from comments.

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

* Ignore negative indices silently so that the behavior of GPU and CPU matches.

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

* Add documentation for the explanation of the negative dropping behavior.

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