tensorflow / tensorflow/quantum

Getting the jacobian inside GradientTape

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Description

If I want to compute the jacobian inside of gradient tape (so I can use it downstream in optimization) classically I can do:

x = tf.random.normal([7, 1])
model = tf.keras.Sequential([
    tf.keras.layers.Input(shape=(1,)),
    tf.keras.layers.Dense(5),
])

with tf.GradientTape(persistent=True) as tape:
    y = model(x)
    j = tape.jacobian(y, model.trainable_variables[0])

But if I try to do that with a PQC in the mode, I get the following error: LookupError: No gradient defined for operation'TfqAdjointGradient' (op type: TfqAdjointGradient). In general every operation must have an associated `@tf.RegisterGradient` for correct autodiff, which this op is lacking. If you want to pretend this operation is a constant in your program, you may insert `tf.stop_gradient`. This can be useful to silence the error in cases where you know gradients are not needed, e.g. the forward pass of tf.custom_gradient. Please see more details in https://www.tensorflow.org/api_docs/python/tf/custom_gradient.

Example code:


x = tfq.convert_to_tensor([cirq.Circuit()] * 7)
qubits = cirq.GridQubit.rect(1, 2)
readouts = [cirq.Z(i) for i in qubits]
s = sympy.symbols("a b")
c = cirq.Circuit()
c += cirq.ry(s[0]).on(qubits[0])
c += cirq.ry(s[1]).on(qubits[1])

model = tf.keras.Sequential([
    tf.keras.layers.Input(shape=(), dtype=tf.string),
    tfq.layers.PQC(c, readouts),
])

with tf.GradientTape(persistent=True) as tape:
    y = model(x)
    j = tape.jacobian(y, model.trainable_variables[0])

If I put in a tf.stop_gradient(y) then I just get the jacobian to be none. Also I don't want to use a stop gradient because there are down stream tasks I want the gradient accumulating through for a final GD update. Is this is a known limitation or am I doing something wrong? In either case, what would your recommended path forward be?

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Research direction

Reproduce the issue with the provided Python example, starting at tfq.layers.PQC and the GradientTape.jacobian call; inspect the reported TfQAdjointGradient operation. Done means the Jacobian can be computed inside the tape while gradients from downstream operations remain available for the final update.

Written by the indexing model from the issue text.

Assessment

Tech stack
python, tensorflow
Domain
machine-learning
Issue type
Bug
Difficulty
4/5
Estimated time
3-5 days
Activity status
Stale
Clarity
Mostly clear
Newbie friendliness
35/100

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