Copy states in Clifford simulator (#3664)

During the (ongoing) review of #3630 95-martin-orion@ found a bug in my draft code. He said that I should copy state like in #3109. I changed my code and added a test in that PR.

However, I had inspired myself from the Clifford code which could have the same issue (I think), and so I am fixing the code in the present PR.

Author: tonybruguier

cirq/sim/clifford/clifford_simulator.py

@@ -225,7 +225,7 @@ def __init__(self, state, measurements):
                 method).
         """
         self.measurements = measurements
-        self.state = state
+        self.state = state.copy()
 
     def __str__(self) -> str:
         def bitstring(vals):

cirq/sim/clifford/clifford_simulator_test.py

@@ -1,3 +1,4 @@
+import itertools
 import numpy as np
 import pytest
 import sympy
@@ -537,3 +538,16 @@ def test_reset():
     assert cirq.CliffordSimulator().sample(c)["out"][0] == 0
     c = cirq.Circuit(cirq.reset(q), cirq.measure(q, key="out"))
     assert cirq.CliffordSimulator().sample(c)["out"][0] == 0
+
+
+def test_state_copy():
+    sim = cirq.CliffordSimulator()
+
+    q = cirq.LineQubit(0)
+    circuit = cirq.Circuit(cirq.H(q), cirq.H(q))
+
+    state_tableaux = []
+    for step in sim.simulate_moment_steps(circuit):
+        state_tableaux.append(step.state.tableau)
+    for x, y in itertools.combinations(state_tableaux, 2):
+        assert not np.shares_memory(x.rs, y.rs)