Remove warnings in tf.losses.softmax_cross_entropy

This fix tries to address the issue raised in 16534 where
tf.losses.softmax_cross_entropy causes warnings due
to the calling of tf.nn.softmax_cross_entropy_with_logits.

This fix switches to tf.nn.softmax_cross_entropy_with_logits_v2
to remove the warning.

This fix fixes 16534.

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