Expose `tf.contrib.layers.conv1d` like conv2d/conv3d

This fix tries to address the issue raised in 20551
where `tf.contrib.layers.conv1d` is not exposed.
In tf.contrib.layers even though conv1d/conv2d/conv3d
are all available, the conv1d was not exposed like
conv2d/conv3d.

This fix exposed `tf.contrib.layers.conv1d` so that
it is consistent with `tf.contrib.layers.conv2d`
and `tf.contrib.layers.conv3d`.

This fix fixes 20551.

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