Add transpile option to disable routing

[kdk]

What is the expected enhancement?

In some contexts, users want to manually route their circuits. If a circuit is already routed, the default transpile levels will skip running the router, but a) this is not publicly documented and b) if a user makes a mistake in their routing, the routing stage will still run (and the user will likely not immediately notice their error).

#3999 added routing_method as a keyword argument to transpile. We should add an argument like routing_method='none' to allow users to specify that the routing should not be run.

[ajavadia]

I gave an example in #3999, which you rightly pointed out won't work.

transpile(circuit, backend, coupling_map=None)

Instead, I think the correct way to this is actually:

transpile(circuit, backend, coupling_map=CouplingMap.from_full(backend.num_qubits+1))

This means assume all the characteristics of that device, just override its coupling map to be fully connected.

[kdk]

Why backend.num_qubits+1? Would this remove our ability to run CXDirection, or raise early if a user mis-routes their circuit?

[1ucian0]

I like routing_method='none'.

[ajavadia]

Sorry just meant backend.num_qubits How does raising early work with routing_method=‘none’? Seems like by choosing that option you are assuming responsibility, and don’t want errors.

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On Apr 1, 2020, at 11:48 AM, Luciano Bello ***@***.***> wrote: I like routing_method='none'. — You are receiving this because you commented. Reply to this email directly, view it on GitHub, or unsubscribe.

[kdk]

If a user selects routing_method='none', we still have a coupling map so we can still run the equivalent of CheckMap as a constraint check or validation pass.