Function gradients

Constructs symbolic derivatives of sum of `ys` w.r.t. x in `xs`. `ys` and `xs` are each a `Tensor` or a list of tensors. `grad_ys` is a list of `Tensor`, holding the gradients received by the `ys`. The list must be the same length as `ys`. `gradients()` adds ops to the graph to output the

(ys,
              xs,
              grad_ys=None,
              name="gradients",
              colocate_gradients_with_ops=False,
              gate_gradients=False,
              aggregation_method=None,
              stop_gradients=None)