Tensorflow gradients

tf.gradients

tf.gradients

tf.gradients(
    ys,
    xs,
    grad_ys=None,
    name='gradients',
    colocate_gradients_with_ops=False,
    gate_gradients=False,
    aggregation_method=None,
    stop_gradients=None
)

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 derivatives of ys with respect to xs. It returns a list of Tensor of length len(xs) where each tensor is the sum(dy/dx) for y in ys.

grad_ys is a list of tensors of the same length as ys that holds the initial gradients for each y in ys. When grad_ys is None, we fill in a tensor of ‘1’s of the shape of y for each y in ys. A user can provide their own initial grad_ys to compute the derivatives using a different initial gradient for each y (e.g., if one wanted to weight the gradient differently for each value in each y).

stop_gradients is a Tensor or a list of tensors to be considered constant with respect to all xs. These tensors will not be backpropagated through, as though they had been explicitly disconnected using stop_gradient. Among other things, this allows computation of partial derivatives as opposed to total derivatives. For example:

a = tf.constant(0.)
b = 2 * a
g = tf.gradients(a + b, [a, b], stop_gradients=[a, b])

Here the partial derivatives g evaluate to [1.0, 1.0], compared to the total derivatives tf.gradients(a + b, [a, b]), which take into account the influence of a on b and evaluate to [3.0, 1.0]. Note that the above is equivalent to:

a = tf.stop_gradient(tf.constant(0.))
b = tf.stop_gradient(2 * a)
g = tf.gradients(a + b, [a, b])

stop_gradients provides a way of stopping gradient after the graph has already been constructed, as compared to tf.stop_gradient which is used during graph construction. When the two approaches are combined, backpropagation stops at both tf.stop_gradient nodes and nodes in stop_gradients, whichever is encountered first.