fix[next]: make past_to_itir cached in default transforms

[DropD]

Description

Make the default transforms use

Requirements

• All fixes and/or new features come with corresponding tests.
• Important design decisions have been documented in the approriate ADR inside the docs/development/ADRs/ folder.

If this PR contains code authored by new contributors please make sure:

• All the authors are covered by a valid contributor assignment agreement provided to ETH Zurich and signed by the employer if needed.
• The PR contains an updated version of the AUTHORS.md file adding the names of all the new contributors.

[tehrengruber]

As discussed the superficial nesting in PastToITIR is terrible, but I can live with that for now. Please add a small todo then we can merge.