`tf.gradients()`.

## Thanks to our Contributors

This release contains contributions from many people at Google, as well as:

a6802739, Aaron Ma, Abdullah Selek, Abolfazl Shahbazi, Ag Ramesh, Albert Z. Guo,
Albin Joy, Alex Itkes, Alex Sergeev, Alexander Pivovarov, Alexey Romanov,
alhkad, Amit Srivastava, amoitra, Andrew Lihonosov, Andrii Prymostka, Anuj