tf.gradients

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Constructs symbolic derivatives of sum of ys w.r.t. x in xs.

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

tf.gradients is only valid in a graph context. In particular, it is valid in the context of a tf.function wrapper, where code is executing as a graph.

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 and for x in xs.

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:

@tf.function
def example():
  a = tf.constant(0.)
  b = 2 * a
  return tf.gradients(a + b, [a, b], stop_gradients=[a, b])
example()
[<tf.Tensor: shape=(), dtype=float32, numpy=1.0>,
<tf.Tensor: shape=(), dtype=float32, numpy=1.0>]

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:

@tf.function
def example():
  a = tf.stop_gradient(tf.constant(0.))
  b = tf.stop_gradient(2 * a)
  return tf.gradients(a + b, [a, b])
example()
[<tf.Tensor: shape=(), dtype=float32, numpy=1.0>,
<tf.Tensor: shape=(), dtype=float32, numpy=1.0>]

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.

All integer tensors are considered constant with respect to all xs, as if they were included in stop_gradients.

unconnected_gradients determines the value returned for each x in xs if it is unconnected in the graph to ys. By default this is None to safeguard against errors. Mathematically these gradients are zero which can be requested using the 'zero' option. tf.UnconnectedGradients provides the following options and behaviors:

@tf.function
def example(use_zero):
  a = tf.ones([1, 2])
  b = tf.ones([3, 1])
  if use_zero:
    return tf.gradients([b], [a], unconnected_gradients='zero')
  else:
    return tf.gradients([b], [a], unconnected_gradients='none')
example(False)
[None]
example(True)
[<tf.Tensor: shape=(1, 2), dtype=float32, numpy=array([[0., 0.]], ...)>]

Let us take one practical example which comes during the back propogation phase. This function is used to evaluate the derivatives of the cost function with respect to Weights Ws and Biases bs. Below sample implementation provides the exaplantion of what it is actually used for :

@tf.function
def example():
  Ws = tf.constant(0.)
  bs = 2 * Ws
  cost = Ws + bs  # This is just an example. Please ignore the formulas.
  g = tf.gradients(cost, [Ws, bs])
  dCost_dW, dCost_db = g
  return dCost_dW, dCost_db
example()
(<tf.Tensor: shape=(), dtype=float32, numpy=3.0>,
<tf.Tensor: shape=(), dtype=float32, numpy=1.0>)