Qcircuit exponent (quantumlib#4593)

Currently the `cirq.contrib.circuit_to_latex_using_qcircuit` utility doesn't handle exponents for multiqubit gates properly. For example, converting `cirq.ISWAP(q0, q1) ** 0.5` outputs the qcircuit source
```
\Qcircuit @r=1em @C=0.75em {
 \\
 &\lstick{\text{0}}& \qw&\multigate{1}{\text{ISWAP}^{0.5}}^0.5 \qw&\qw\\
 &\lstick{\text{1}}& \qw&\ghost{\text{ISWAP}^{0.5}}^0.5        \qw&\qw\\
 \\
}
```
which produces the diagram
![before](https://user-images.githubusercontent.com/14928221/138499001-8fe6c0d3-f20a-43e3-be87-88e7650d93b2.png)

The `0.5` exponent is getting duplicated and put in the wrong place. 

This PR resolves this issue by bypassing any additional processing by `Circuit.to_text_diagram_drawer` by just hardcoding `exponent=1` into `CircuitDiagramInfo`. This results in the correct output,
```
\Qcircuit @r=1em @C=0.75em {
 \\
 &\lstick{\text{0}}& \qw&\multigate{1}{\text{ISWAP}^{0.5}} \qw&\qw\\
 &\lstick{\text{1}}& \qw&\ghost{\text{ISWAP}^{0.5}}        \qw&\qw\\
 \\
}
```
and the diagram
![after](https://user-images.githubusercontent.com/14928221/138499507-d86a781d-5013-4871-b9e5-46f364f032c0.png)

cirq/contrib/qcircuit/qcircuit_diagram_info.py

@@ -98,9 +98,8 @@ def multigate_qcircuit_diagram_info(
     assert args.qubit_map is not None
     assert args.known_qubits is not None
     symbols = tuple(box if (args.qubit_map[q] == min_index) else ghost for q in args.known_qubits)
-    return protocols.CircuitDiagramInfo(
-        symbols, exponent=1 if info is None else info.exponent, connected=False
-    )
+    # Force exponent=1 to defer to exponent formatting given above.
+    return protocols.CircuitDiagramInfo(symbols, connected=False)
 
 
 def fallback_qcircuit_diagram_info(

cirq/contrib/qcircuit/qcircuit_diagram_info_test.py

@@ -32,7 +32,7 @@ def test_get_qcircuit_diagram_info():
     actual_info = ccq.get_qcircuit_diagram_info(op, args)
     name = r'{\text{SWAP}^{0.5}}'
     expected_info = cirq.CircuitDiagramInfo(
-        (r'\multigate{1}' + name, r'\ghost' + name), exponent=0.5, connected=False
+        (r'\multigate{1}' + name, r'\ghost' + name), exponent=1, connected=False
     )
     assert actual_info == expected_info
 

cirq/contrib/qcircuit/qcircuit_test.py

@@ -155,3 +155,17 @@ def test_two_cx_diagram():
  \\
 }""".strip()
     assert_has_qcircuit_diagram(circuit, expected_diagram)
+
+
+def test_sqrt_iswap_diagram():
+    # test for proper rendering of ISWAP^{0.5}
+    q0, q1 = cirq.LineQubit.range(2)
+    circuit = cirq.Circuit(cirq.ISWAP(q0, q1) ** 0.5)
+    expected_diagram = r"""
+\Qcircuit @R=1em @C=0.75em {
+ \\
+ &\lstick{\text{0}}& \qw&\multigate{1}{\text{ISWAP}^{0.5}} \qw&\qw\\
+ &\lstick{\text{1}}& \qw&\ghost{\text{ISWAP}^{0.5}}        \qw&\qw\\
+ \\
+}""".strip()
+    assert_has_qcircuit_diagram(circuit, expected_diagram)