GridDevice should allow measuring any qubit

[maffoo]

Description of the issue

Currently cirq_google.GridDevice requires that for any operation with exactly two qubits, the pair of qubits must be in qubit_pairs. However, not all 2-qubit operations require the qubits to be coupled. In particular, MeasurementGate should be allowed on any set of qubits, including pairs that are not coupled.

How to reproduce the issue

In [1]: import cirq; import cirq_google

In [2]: cirq_google.Sycamore.validate_operation(cirq.measure(cirq.q(0, 5), cirq.q(2, 5), cirq.q(4, 5)))

In [3]: cirq_google.Sycamore.validate_operation(cirq.measure(cirq.q(0, 5), cirq.q(2, 5)))
---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)
<ipython-input-5-a955df2ee85c> in <module>
----> 1 cirq_google.Sycamore.validate_operation(cirq.measure(cirq.q(0, 5), cirq.q(2, 5)))

~/src/quantumlib/cirq/cirq-google/cirq_google/devices/grid_device.py in validate_operation(self, operation)
    341             and frozenset(operation.qubits) not in self._metadata.qubit_pairs
    342         ):
--> 343             raise ValueError(f'Qubit pair is not valid on device: {operation.qubits!r}.')
    344 
    345     def __str__(self) -> str:

ValueError: Qubit pair is not valid on device: (cirq.GridQubit(0, 5), cirq.GridQubit(2, 5)).

Cirq version

0.15

[verult]

Thanks for the catch!