Add complex support for tf.segment_mean (#19225)

* Add complex support for tf.segment_mean

While using tf.segment_mean I noticed that it does not
have the complex support like tf.segment_sum. I think it
makes sense to support complex for it. This fix adds the
complex support for tf.segment_mean.

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

* Add test cases for complex support with tf.segment_mean

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