Add unspecified dimensions (-1) support for noise_shape with tf.nn.dropout

This fix tries to address the issue raised in 16034 where
it was not possible to have unspecified dimensions
for `noise_shape` with `tf.nn.dropout`.

This fix adds the support so that it is possible to specify
`noise_shape = [-1, 1, 1, -1]` instead of `noise_shape = [k, 1, 1, n]`.

This fix fixes 16034.

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