to change uses of lists to tuples in existing code. In addition, dicts are
    now supported as a nested structure.

## Changes to contrib APIs

*   Adds tf.contrib.nn.rank_sampled_softmax_loss, a sampled-softmax variant that
    can improve rank loss.
*   `tf.contrib.metrics`.{streaming_covariance,streaming_pearson_correlation}
    modified to return nan when they have seen less or equal to 1 unit of