Remove qubit pair ordering requirement in SerializableDevice

[verult]

This call site was missed after #5169 was fixed.

If gate_definitions contain a pair of the same qubit, GridDeviceMetadata will throw a ValueError.

@mpharrigan

[mpharrigan]

Sorry, can you describe what was happening here and what should be happening here? Was this a latent bug? What this pair just not added to the device topology because the constructor expects ordered pairs but got unordered pairs? What defense do you have against passing unordered pairs to the GridMetadata constructor?

[verult]

Looked into this again, and turns out the previous code was fine. For each SYMMETRIC target in the DeviceSpecification proto, two tuples are created in the gate definition, one for each direction of the pair:

Cirq/cirq-google/cirq_google/devices/serializable_device.py

Line 223 in 1769cba

target_set.add(qid_tuple[::-1])

so the pair[0] < pair[1] deduplicates the pair, and no qubit pairs should be omitted.

I'll go ahead and close this PR (including the test case change), since SerializableDevice will be removed soon. But let me know if you feel otherwise.

What defense do you have against passing unordered pairs to the GridMetadata constructor?

In the current GridDeviceMetadata implementation, qubit pairs passed into the constructor are considered to be unordered

[mpharrigan]

That works as long as you're coming via that proto code path, but if you're just using the bare __init__ method, could you get yourself into hot water?

[verult]

Yeah that is true. I am OK either way since the preferred method to construct SerializableDevice is from_proto() (as mentioned in the __init__ docstring).

[mpharrigan]

that seems dangerous to me.

When is this class getting removed?

[mpharrigan]

bump

[verult]

This class is now gone in Cirq 1.0. PR: #5743

[pavoljuhas]

I did not dig into the detail, but the updated test passes with the unchanged serializable_device.py module.
Perhaps the test needs to be adjusted?

[verult]

Closing (rationale in #5642 (comment))