Fix diagram for Identity with no qubits (quantumlib#6926)

* Fix diagram for Identity with no qubits

- An IdentityGate with 0 qubits used to raise an exception
in the circuit drawer.
- This PR fixes the issue and displays the gate similar to a
GlobalPhaseGate.

Fixes: quantumlib#6768

cirq-core/cirq/ops/identity.py

@@ -135,6 +135,8 @@ def _mul_with_qubits(self, qubits: Tuple['cirq.Qid', ...], other):
     _rmul_with_qubits = _mul_with_qubits
 
     def _circuit_diagram_info_(self, args) -> Tuple[str, ...]:
+        if self.num_qubits() <= 0:
+            return NotImplemented
         return ('I',) * self.num_qubits()
 
     def _qasm_(self, args: 'cirq.QasmArgs', qubits: Tuple['cirq.Qid', ...]) -> Optional[str]:

cirq-core/cirq/ops/identity_test.py

@@ -214,3 +214,21 @@ def test_identity_commutes():
     assert cirq.commutes(cirq.I, cirq.X)
     with pytest.raises(TypeError):
         cirq.commutes(cirq.I, "Gate")
+
+
+def test_identity_diagram():
+    cirq.testing.assert_has_diagram(
+        cirq.Circuit(cirq.IdentityGate(3).on_each(cirq.LineQubit.range(3))),
+        """
+0: ───I───
+      │
+1: ───I───
+      │
+2: ───I───
+""",
+    )
+    cirq.testing.assert_has_diagram(
+        cirq.Circuit(cirq.IdentityGate(0)()),
+        """
+    I(0)""",
+    )

cirq-core/cirq/protocols/circuit_diagram_info_protocol.py

@@ -342,7 +342,7 @@ def _op_info_with_fallback(
     rows: List[LabelEntity] = list(op.qubits)
     if args.label_map is not None:
         rows += protocols.measurement_keys_touched(op) & args.label_map.keys()
-    if info is not None:
+    if info is not None and info.wire_symbols:
         if max(1, len(rows)) != len(info.wire_symbols):
             raise ValueError(f'Wanted diagram info from {op!r} for {rows!r}) but got {info!r}')
         return info