Fix issue in tf.nn.softmax where negative dims could only be -1

This fix tries to address the issue raised in 14916 where
negative dims could only be -1 in tf.nn.softmax.
The issue was that dims=-1 was handled as a case of "last dim"
with `is_last_dim = (dim is -1) or (dim == shape.ndims - 1)`
but the generic negative dims were never processed.

This fix adds `dim += shape.ndims` for generic negative dims.

This fix fixes 14916.

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