*   Other:

    *   Fix accidental quadratic graph construction cost in graph-mode
        `tf.gradients()`.
    *   ResourceVariable's gather op supports batch dimensions.
    *   ResourceVariable support for `gather_nd`.
    *   `ResourceVariable` and `Variable` no longer accepts `constraint` in the
        constructor, nor expose it as a @property.
    *   Added gradient for `SparseToDense` op.