Fix VQC training with warm_start=True (#312)

* fixes #296 warm_start with vqc

extract the initial_point in TrainableModel from the final point of the
minimization in the existing OptimizerResult object.

* update copyright

* add units tests to check vqc warm_start

* fix grammar

* fix grammar

* initialise -> initialize

* fix typo in test docstring

* add a reno for fix of #296

* fix quotes in reno report

Co-authored-by: Anton Dekusar <62334182+adekusar-drl@users.noreply.github.com>
(cherry picked from commit 5a02c76)

qiskit_machine_learning/algorithms/trainable_model.py

@@ -1,6 +1,6 @@
 # This code is part of Qiskit.
 #
-# (C) Copyright IBM 2021.
+# (C) Copyright IBM 2021, 2022.
 #
 # This code is licensed under the Apache License, Version 2.0. You may
 # obtain a copy of this license in the LICENSE.txt file in the root directory
@@ -201,7 +201,7 @@ def _choose_initial_point(self) -> np.ndarray:
             An array as an initial point
         """
         if self._warm_start and self._fit_result is not None:
-            self._initial_point = self._fit_result[0]
+            self._initial_point = self._fit_result.x
         elif self._initial_point is None:
             self._initial_point = algorithm_globals.random.random(self._neural_network.num_weights)
         return self._initial_point

releasenotes/notes/fix-vqc-warm-start-1c15d3f5742c5e84.yaml

@@ -0,0 +1,11 @@
+---
+fixes:
+  - |
+    Fixes an issue where ``VQC`` would fail with ``warm_start=True``.
+    The extraction of the ``initial_point`` in ``TrainableModel`` from the final
+    point of the minimization had not been updated to reflect the refactor
+    of optimizers in ``qiskit-terra``; the old ``optimize`` method, that returned
+    a tuple was deprecated and new method ``minimize`` was created that returns
+    an ``OptimizerResult`` object. We now correctly recover the final point of
+    the minimization from previous fits to use for a warm start in subsequent
+    fits.

test/algorithms/classifiers/test_vqc.py

@@ -230,6 +230,70 @@ def test_multiclass(self, config):
         score = classifier.score(X, y)
         self.assertGreater(score, 1 / num_classes)
 
+    @data(
+        # optimizer, quantum instance
+        ("cobyla", "statevector"),
+        ("cobyla", "qasm"),
+        ("bfgs", "statevector"),
+        ("bfgs", "qasm"),
+        (None, "statevector"),
+        (None, "qasm"),
+    )
+    def test_warm_start(self, config):
+        """Test VQC with warm_start=True."""
+        opt, q_i = config
+
+        if q_i == "statevector":
+            quantum_instance = self.sv_quantum_instance
+        else:
+            quantum_instance = self.qasm_quantum_instance
+
+        if opt == "bfgs":
+            optimizer = L_BFGS_B(maxiter=5)
+        elif opt == "cobyla":
+            optimizer = COBYLA(maxiter=25)
+        else:
+            optimizer = None
+
+        num_inputs = 2
+        feature_map = ZZFeatureMap(num_inputs)
+        ansatz = RealAmplitudes(num_inputs, reps=1)
+
+        # Construct the data.
+        num_samples = 10
+        # pylint: disable=invalid-name
+        X = algorithm_globals.random.random((num_samples, num_inputs))
+        y = 1.0 * (np.sum(X, axis=1) <= 1)
+        while len(np.unique(y)) == 1:
+            X = algorithm_globals.random.random((num_samples, num_inputs))
+            y = 1.0 * (np.sum(X, axis=1) <= 1)
+        y = np.array([y, 1 - y]).transpose()  # VQC requires one-hot encoded input.
+
+        # Initialize the VQC.
+        classifier = VQC(
+            feature_map=feature_map,
+            ansatz=ansatz,
+            optimizer=optimizer,
+            warm_start=True,
+            quantum_instance=quantum_instance,
+        )
+
+        # Fit the VQC to the first half of the data.
+        num_start = num_samples // 2
+        classifier.fit(X[:num_start, :], y[:num_start])
+        first_fit_final_point = classifier._fit_result.x
+
+        # Fit the VQC to the second half of the data with a warm start.
+        classifier.fit(X[num_start:, :], y[num_start:])
+        second_fit_initial_point = classifier._initial_point
+
+        # Check the final optimization point from the first fit was used to start the second fit.
+        np.testing.assert_allclose(first_fit_final_point, second_fit_initial_point)
+
+        # Check score.
+        score = classifier.score(X, y)
+        self.assertGreater(score, 0.5)
+
 
 if __name__ == "__main__":
     unittest.main()