Removing noise model check in Simulator initialization (#4216)

Addresses issue #4170 by removing NOISE_MODEL_LIKE check in sparse_simulator.py and updates tests accordingly.

cirq-core/cirq/sim/sparse_simulator.py

@@ -27,7 +27,7 @@
 
 import numpy as np
 
-from cirq import ops, protocols, qis, devices
+from cirq import ops, protocols, qis
 from cirq.sim import (
     simulator,
     state_vector,
@@ -154,9 +154,6 @@ def __init__(
         """
         if np.dtype(dtype).kind != 'c':
             raise ValueError(f'dtype must be a complex type but was {dtype}')
-        noise_model = devices.NoiseModel.from_noise_model_like(noise)
-        if not protocols.has_mixture(noise_model):
-            raise ValueError(f'noise must be unitary or mixture but was {noise_model}')
         super().__init__(
             dtype=dtype,
             noise=noise,

cirq-core/cirq/sim/sparse_simulator_test.py

@@ -1270,6 +1270,12 @@ def test_nondeterministic_mixture_noise():
     assert result1 != result2
 
 
-def test_unsupported_noise_fails():
-    with pytest.raises(ValueError, match='noise'):
-        cirq.Simulator(noise=cirq.amplitude_damp(0.5))
+def test_noise_model():
+    q = cirq.LineQubit(0)
+    circuit = cirq.Circuit(cirq.H(q), cirq.measure(q))
+
+    noise_model = cirq.NoiseModel.from_noise_model_like(cirq.depolarize(p=0.01))
+    simulator = cirq.Simulator(noise=noise_model)
+    result = simulator.run(circuit, repetitions=100)
+
+    assert 40 <= sum(result.measurements['0'])[0] < 60