Fix deprecated keyword argument in dropout layer.

[jarednielsen]

The function signature for tf.nn.dropout in TF1 is:

tf.nn.dropout(
    x,
    keep_prob=None,
    noise_shape=None,
    seed=None,
    name=None,
    rate=None
)

while TF2 has:

tf.nn.dropout(
    x, rate, noise_shape=None, seed=None, name=None
)

Using the rate keyword argument is compatible with both, and less confusing.

This is done in the ALBERT repo: https://github.com/google-research/albert/blob/c21d8a3616a4b156d21e795698ad52743ccd8b73/modeling.py#L412-L427