Creating a challenging backend to show the transpiler stochasticity

[1ucian0]

The documentation about the transpiler shows that the process is stochastic in https://docs.quantum.ibm.com/api/qiskit/transpiler#routing-stage:

In order to highlight this, we run a GHZ circuit 100 times, using a “bad” (disconnected) initial_layout:


This distribution is quite wide, signaling the difficulty the swap mapper is having in computing the best mapping.

Which is not very wide. That's because GenericFakeBackend replaced a FakeBackend in #11376. The GenericFakeBackend has an all-to-all connectivity by default.

This PR produces the following image:

[qiskit-bot]

One or more of the the following people are requested to review this:

• @Qiskit/terra-core

[coveralls]

Pull Request Test Coverage Report for Build 8522176991

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 2 unchanged lines in 1 file lost coverage.
• Overall coverage increased (+0.003%) to 89.341%

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Files with Coverage Reduction	New Missed Lines	%
crates/qasm2/src/lex.rs	2	92.37%

Totals
Change from base Build 8521949858:	0.003%
Covered Lines:	60257
Relevant Lines:	67446

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💛 - Coveralls

[jakelishman]

This seems like a logical improvement to the docs to me - thanks for fixing it!