Fix batch_execute output type for QNodes with multiple outputs (#163)

* asarray

* check type

* changelog

CHANGELOG.md

@@ -2,6 +2,13 @@
 
 ### New features since last release
 
+* Defined the `QiskitDevice.batch_execute` method, to allow
+  Qiskit backends to run multiple quantum circuits at the same time. This
+  addition allows submitting batches of circuits to IBMQ e.g., when computing
+  gradients internally.
+  [(#156)](https://github.com/PennyLaneAI/pennylane-qiskit/pull/156)
+  [(#163)](https://github.com/PennyLaneAI/pennylane-qiskit/pull/163)
+
 ### Breaking changes
 
 ### Improvements
@@ -12,7 +19,8 @@
 * Added support for the `qml.SX` operation to the Qiskit devices.
   [(#158)](https://github.com/PennyLaneAI/pennylane-qiskit/pull/158)
 
-* Added support for returning job execution times. [(#160)](https://github.com/PennyLaneAI/pennylane-qiskit/pull/160)
+* Added support for returning job execution times.
+  [(#160)](https://github.com/PennyLaneAI/pennylane-qiskit/pull/160)
 
 ### Documentation
 

pennylane_qiskit/qiskit_device.py

@@ -409,6 +409,7 @@ def batch_execute(self, circuits):
                 self._samples = self.generate_samples(circuit_obj)
 
             res = self.statistics(circuit.observables)
+            res = np.asarray(res)
             results.append(res)
 
         if self.tracker.active:

tests/test_qiskit_device.py

@@ -155,8 +155,10 @@ def test_result(self, device, tol):
         tape2_expected = dev.execute(self.tape2)
 
         assert len(res) == 2
+        assert isinstance(res[0], np.ndarray)
         assert np.allclose(res[0], tape1_expected, atol=0)
 
+        assert isinstance(res[1], np.ndarray)
         assert np.allclose(res[1], tape2_expected, atol=0)
 
     def test_result_empty_tape(self, device, tol):