Make sure PEC preserves measurement gates (unitaryfund#1844)

* fix bug

* check last get is measurement in tests

mitiq/pec/sampling.py

@@ -149,19 +149,13 @@ def sample_circuit(
     norm = 1.0
 
     for op in ideal.all_operations():
-        # Ignore all measurements.
-        if cirq.is_measurement(op):
-            continue
-
         sequences, loc_signs, loc_norm = sample_sequence(
             cirq.Circuit(op),
             representations,
             num_samples=num_samples,
             random_state=random_state,
         )
-
         norm *= loc_norm
-
         for j in range(num_samples):
             sampled_signs[j] *= loc_signs[j]
             cirq_seq, _ = convert_to_mitiq(sequences[j])

mitiq/pec/tests/test_pec_sampling.py

@@ -147,7 +147,10 @@ def test_qubit_independent_representation_cirq():
     )
 
     expected_a = Circuit([cirq.I.on(LineQubit(0)), cirq.X.on(LineQubit(1))])
+    expected_a.append(measure_each(*LineQubit.range(2)))
+
     expected_b = Circuit([cirq.I.on(LineQubit(0)), cirq.Z.on(LineQubit(1))])
+    expected_b.append(measure_each(*LineQubit.range(2)))
 
     for _ in range(5):
         seqs, signs, norm = sample_circuit(circuit, representations=[rep])
@@ -272,9 +275,15 @@ def test_sample_circuit_cirq(measure):
         )
 
         assert isinstance(sampled_circuits[0], Circuit)
-        assert len(sampled_circuits[0]) == 2
         assert signs[0] in (-1, 1)
         assert norm >= 1
+        if measure:
+            assert len(sampled_circuits[0]) == 3
+            assert cirq.is_measurement(
+                list(sampled_circuits[0].all_operations())[-1]  # last gate
+            )
+        else:
+            assert len(sampled_circuits[0]) == 2
 
 
 def test_sample_circuit_partial_representations():