introduce default_passmanager to replace generate_preset_pass_manager

[1ucian0]

Summary

This PR introduces default_passmanager which is a cleaner version of generate_preset_pass_manager, designated to replace transpile in 2.0

Notes:

• the broken test could be because Update target from_configuration #11461 ?

[qiskit-bot]

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

• @Qiskit/terra-core

[mtreinish]

I'm going to close this PR as I don't think we're pursuing this direction anymore. We re-exported generate_preset_pass_manager from the root qiskit module and also set 2 as the default value for optimization level. So users can do:

from qiskit import generate_preset_pass_manager
generate_preset_pass_manager(backend)

or

from qiskit import generate_preset_pass_manager
generate_preset_pass_manager(backend.target)

which I think covers the use case this was trying to address which was to lower the barrier to constructing a preset pass manager using a good default.