Function gradients_v2

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,  # pylint: disable=invalid-name
                 xs,
                 grad_ys=None,
                 name="gradients",
                 gate_gradients=False,
                 aggregation_method=None,
                 stop_gradients=None,
                 unconnected_gradients=UnconnectedGradients.NONE)