Add discussion of custom-pass implementation #14787
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This is the beginnings of better documentation around writing custom transpiler passes. Historically, we've somewhat implicitly assumed that people would look at the source code to get a feel for it, but *really*, this should always have been in guide-level documentation. This commit adds some guiding information about the expectations of passes. It doesn't currently add worked examples; these should be added at some point. The primary motivation for this commit was to add the discussion about expectations around determinism of passes, and tips for how to achieve that.
jakelishman
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Summary
This is the beginnings of better documentation around writing custom transpiler passes. Historically, we've somewhat implicitly assumed that people would look at the source code to get a feel for it, but really, this should always have been in guide-level documentation.
This commit adds some guiding information about the expectations of passes. It doesn't currently add worked examples; these should be added at some point.
The primary motivation for this commit was to add the discussion about expectations around determinism of passes, and tips for how to achieve that.
Details and comments
In particular, this commit makes explicit the assumptions that lead to the use of
DAGCircuit.structurally_equal, proposed in #14762. The determinism of Qiskit preset pipelines have largely already relied on the behaviours expounded on here, so this is not intended to be new restrictions, but rather an explicit statement of previously unsaid ones.