Deprecate MergeNQubitGates optimizer (#5653)

Part of #5028

Author: tanujkhattar

cirq-core/cirq/contrib/quimb/Contract-a-Grid-Circuit.ipynb

@@ -206,8 +206,8 @@
    "metadata": {},
    "outputs": [],
    "source": [
-    "ccq.MergeNQubitGates(n_qubits=2).optimize_circuit(compressed_c)\n",
-    "ccq.MergeNQubitGates(n_qubits=1).optimize_circuit(compressed_c)\n",
+    "compressed_c= cirq.merge_k_qubit_unitaries(compressed_c, k=2)\n",
+    "compressed_c = cirq.merge_k_qubit_unitaries(compressed_c, k=1)\n",
     "\n",
     "compressed_c = cirq.drop_negligible_operations(compressed_c, atol=1e-6)\n",
     "compressed_c = cirq.drop_empty_moments(compressed_c)\n",

cirq-core/cirq/contrib/quimb/grid_circuits.py

@@ -89,6 +89,7 @@ def _interaction(
     )
 
 
+@cirq._compat.deprecated_class(deadline='v0.16', fix="Use cirq.merge_k_qubit_unitaries")
 class MergeNQubitGates(cirq.PointOptimizer):
     """Optimizes runs of adjacent unitary n-qubit operations."""
 
@@ -131,14 +132,14 @@ def simplify_expectation_value_circuit(circuit_sand: cirq.Circuit):
     things for you.
     """
     n_op = sum(1 for _ in circuit_sand.all_operations())
+    circuit = circuit_sand.copy()
     while True:
-        MergeNQubitGates(n_qubits=1).optimize_circuit(circuit_sand)
-        circuit_sand = cirq.drop_negligible_operations(circuit_sand, atol=1e-6)
-        MergeNQubitGates(n_qubits=2).optimize_circuit(circuit_sand)
-        circuit_sand = cirq.drop_empty_moments(circuit_sand)
-        new_n_op = sum(1 for _ in circuit_sand.all_operations())
-
-        if new_n_op < n_op:
-            n_op = new_n_op
-        else:
-            return
+        circuit = cirq.merge_k_qubit_unitaries(circuit, k=1)
+        circuit = cirq.drop_negligible_operations(circuit, atol=1e-6)
+        circuit = cirq.merge_k_qubit_unitaries(circuit, k=2)
+        circuit = cirq.drop_empty_moments(circuit)
+        new_n_op = sum(1 for _ in circuit.all_operations())
+        if new_n_op >= n_op:
+            break
+        n_op = new_n_op
+    circuit_sand._moments = circuit._moments