Performance improvements for opflow (qiskit-community/qiskit-aqua#925)

* Performance improvements:
1) Make CircuitOp composition depend on combine
2) Make cache check in CircuitSampler check id(), not deep equality
3) Make to_circuit with measurement copy and measure, rather than insert into new circuit as instruction.

* 1) Fix bug causing CircuitSampler to retranspile over and over.
2) Throw an error if QuantumCircuit with classical bits passed into CircuitStateFn or CircuitOp.

* Move ClassicalRegister import.

* fix IBMQ unit test imports

Co-authored-by: Manoel Marques <[email protected]>

Author: dongreenberg

test/aqua/operators/test_pauli_expectation.py

@@ -25,7 +25,7 @@
                                    PauliExpectation, AbelianGrouper,
                                    CircuitSampler)
 
-from qiskit import BasicAer, IBMQ
+from qiskit import BasicAer
 
 
 # pylint: disable=invalid-name
@@ -190,6 +190,7 @@ def test_grouped_pauli_expectation(self):
     @unittest.skip(reason="IBMQ testing not available in general.")
     def test_ibmq_grouped_pauli_expectation(self):
         """ pauli expect op vector state vector test """
+        from qiskit import IBMQ
         p = IBMQ.load_account()
         backend = p.get_backend('ibmq_qasm_simulator')
         paulis_op = ListOp([X, Y, Z, I])

test/aqua/test_vqe.py

@@ -18,7 +18,7 @@
 from test.aqua import QiskitAquaTestCase
 import numpy as np
 from ddt import ddt, unpack, data
-from qiskit import BasicAer, QuantumCircuit, IBMQ
+from qiskit import BasicAer, QuantumCircuit
 from qiskit.circuit import ParameterVector
 
 from qiskit.aqua import QuantumInstance, aqua_globals, AquaError
@@ -259,6 +259,7 @@ def test_vqe_mes(self):
     @unittest.skip(reason="IBMQ testing not available in general.")
     def test_ibmq_vqe(self):
         """ IBMQ VQE Test """
+        from qiskit import IBMQ
         provider = IBMQ.load_account()
         backend = provider.get_backend('ibmq_qasm_simulator')
         var_form = RYRZ(self.qubit_op.num_qubits)