Add rate to v1 dropout.

This change does change observable behavior in the case where one passes a tf.constant with value 1 to keep_prob. Before, this returned the input (y is x), now it will execute the actual dropout (even though it's pointless) since the input is transformed, and constant_value will not recognize the true value of rate=0 (since it runs before constant folding).

RELNOTES: Dropout now takes `rate` argument, keep_prob is deprecated.
PiperOrigin-RevId: 223609371