Adjust imports in aqc plugin to avoid discovery overhead (#7327)

This commit tweaks the imports for the aqc plugin to make the optimizer
and other aqc method imports occur at run time only. Previously these
were done at the module level, which is normally best practice for many
reasons, but in this specific case doing plugin discovery causes the
module to be loaded (and the plugin class instaniated) even if the
plugin is not specified by the user. The imports for aqc are fairly
heavy as both the algorithms tree and scipy can be slow to load. To
avoid the overhead on discovery these imports are moved to runtime so we
only load them if the aqc plugin is used and these imports are needed.

Co-authored-by: Jake Lishman <[email protected]>

qiskit/transpiler/synthesis/aqc/aqc_plugin.py

@@ -14,13 +14,8 @@
 """
 import numpy as np
 
-from qiskit.algorithms.optimizers import L_BFGS_B
 from qiskit.converters import circuit_to_dag
 from qiskit.transpiler.passes.synthesis.plugin import UnitarySynthesisPlugin
-from qiskit.transpiler.synthesis.aqc.aqc import AQC
-from qiskit.transpiler.synthesis.aqc.cnot_structures import make_cnot_network
-from qiskit.transpiler.synthesis.aqc.cnot_unit_circuit import CNOTUnitCircuit
-from qiskit.transpiler.synthesis.aqc.cnot_unit_objective import DefaultCNOTUnitObjective
 
 
 class AQCSynthesisPlugin(UnitarySynthesisPlugin):
@@ -96,6 +91,15 @@ def supports_coupling_map(self):
         return False
 
     def run(self, unitary, **options):
+
+        # Runtime imports to avoid the overhead of these imports for
+        # plugin discovery and only use them if the plugin is run/used
+        from qiskit.algorithms.optimizers import L_BFGS_B
+        from qiskit.transpiler.synthesis.aqc.aqc import AQC
+        from qiskit.transpiler.synthesis.aqc.cnot_structures import make_cnot_network
+        from qiskit.transpiler.synthesis.aqc.cnot_unit_circuit import CNOTUnitCircuit
+        from qiskit.transpiler.synthesis.aqc.cnot_unit_objective import DefaultCNOTUnitObjective
+
         num_qubits = int(round(np.log2(unitary.shape[0])))
 
         config = options.get("config") or {}