*  Fix: Change the declaration of `OkHttpClient.cache()` to return a `@Nullable Cache`. The return
    value has always been nullable but it wasn't declared properly.
 *  Fix: Reverse suppression of connect exceptions. When both a call and its retry fail, we now
    throw the initial exception which is most likely to be actionable.

*   Improved convolution padding documentation.
*   Add a tag constant, gpu, to present graph with GPU support.
*   `saved_model.utils` now support SparseTensors transparently.
*   A more efficient implementation of non-max suppression.
*   Add support for the shrinkage-type L2 to FtrlOptimizer in addition to the
    online L2 it already supports.
*   Fix negative variance in moments calculation.