functions. AutoGraph is also applied in functions used with `tf.data`,
    `tf.distribute` and `tf.keras` APIS.
*   Adds `enable_tensor_equality()`, which switches the behavior such that:
    *   Tensors are no longer hashable.
    *   Tensors can be compared with `==` and `!=`, yielding a Boolean Tensor
        with element-wise comparison results. This will be the default behavior
        in 2.0.

## Breaking Changes

//
// Gradient is the type returned by gradient functions. In Python TF it's either
// Tensor or IndexedSlices or None, which here we map to nullptr. Gradients need
// to allow their size to be computed and they need to be passable to a backward
// function and deleted (as the backprop code creates lots of gradients the user
// is not interested in).
//
// BackwardFunction needs to be a closure which stores intermediate activations