*   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.
    *   Expose a flag that allows the number of threads to vary across Python
        benchmarks.
    *   `image.resize` in 2.0 now supports gradients for the new resize kernels.