Add generate_clifford_t_pass_manager

[alexanderivrii]

Summary

This PR introduces a function generate_clifford_t_pass_manager which is analogous to generate_preset_pass_manager but exposes arguments specifically tailored for Clifford+T compilation. In particular it does not have arguments such as init_stage or optimization_stage but has arguments rz_synthesis_error and rz_cache_error relevant to synthesizing RZ-gates to Clifford+T basis set.

The previously existing ways of instantiating a Clifford+T transpiler pipeline (via transpile or generate_preset_pass_manager with a Clifford+T basis set) are still supported, however using the new function is recommended.

Details and comments

I have defined the following classes:

• PassManagerCliffordTConfig, which is analogous to PassManagerConfig but tailored for Clifford+T. Currently it subclasses PassManagerConfig as Clifford+T compilation pipeline (not changed in this PR) uses default layout, routing and scheduling stages. (Actually the only place which really needs this to be a subclass is DefaultRoutingPassManager which calls SabreSwapPassManager, which assumes that config includes layout_method. And I did not want to explicitly add layout_method to PassManagerCliffordTConfig as per previous discussions we might want to completely change how layout and routing look for Clifford+T compilations).

• PassManagerCliffordTStagePlugin, which is analogous to PassManagerCliffordTStagePlugin, but tailored for Clifford+T compilation.

Both of the above classes are internal for now (not exposed in the documentation on purpose) since we might want to further refactor them when the PBC compilation pipeline will be added.

[qiskit-bot]

One or more of the following people are relevant to this code:

• @Qiskit/terra-core

[Cryoris]

I would expect this to go under the umbrella of generic synthesis arguments. Once we add a plugin here we'll need to accommodate for this, plus other synthesis methods might handle caching differently. Could we just call this rz_synthesis_config and make it a generic dictionary to pass into the synthesis method?

[alexanderivrii]

That was mainly to show that we now can add Clifford+T pipeline-specific arguments (and because you have asked for this for comparing against other tools). But I agree that we need to think a bit more about how synthesis and Clifford+T should look.

[Cryoris]

Some high level comments, I didn't read the logic yet 🙂

[Cryoris]

Could we ditch these and just use a Target instead? Or is there a reason to use these as standalone?

[alexanderivrii]

In theory, we can, however I am personally so used to generate preset pass managers while specifying basis_gates and coupling_map manually, that I would miss this functionality. However, based on our offline discussion, I have removed backend as an argument (especially that we have no such backends at the moment).