In smallpebble it's currently done by setting the value of the Placeholders, e.g.

    # define some graph
    x = sp.Placeholder()
    y = sp.Lazy(sp.square)(x)

    # and run the graph later on
    x.assign_value(sp.Variable(np.array([1, 2, 3])))
    y_val = y.run()  # the result, y_val is an sp.Variable
    gradients = sp.get_gradients(y_val)

    # run same graph with different values
    x.assign_value(sp.Variable(np.array([5, 6, 7])))
    y_val = y.run()
    gradients = sp.get_gradients(y_val)

(There's some more examples of this being used in training loops in the readme.)