redefining OperatorEvolution class and fixing typehints

[lballerio]

In this PR, I am redefining the emulator's data handling to make it clearer and more expressive, particularly the way static and time-dependent Hamiltonians are represented before starting the simulation.

Specifically, the static and dynamical contributions of the full Hamiltonian have been split into two separate objects: the static term is now represented by an Operator, while the dynamical terms are represented by an OperatorEvolution.

The main issue with the previous design was that OperatorEvolution.operators is typed as list[Operator | TimeDependentOperator], which introduces some ambiguity. In particular, when loading data from the filesystem, main currently relies on the convention that the first element of this list is the static term. I believe the new design makes the data model much clearer by explicitly separating static and dynamical terms instead of encoding them within a single object.

To better understand the motivation behind these changes, please also refer to the updates to the loading functions in qutip.py.

Finally, I added several type hints throughout the code.

Note that we have decided to focus exclusively on the qutip engine. Therefore, feedback related to the dynamiqs engine is not a priority, as it may already be outdated, partially broken, and not fully tested.

Checklist:

• Reviewers confirm new code works as expected.
• Tests are passing.
• Coverage does not decrease.
• Documentation is updated.

[codecov]

Codecov Report

❌ Patch coverage is 93.75000% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 38.96%. Comparing base (dd6f942) to head (339f6dd).

Files with missing lines	Patch %	Lines
...qibolab/_core/instruments/emulator/engine/qutip.py	66.66%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1528      +/-   ##
==========================================
+ Coverage   38.90%   38.96%   +0.05%     
==========================================
  Files         120      120              
  Lines        6225     6231       +6     
==========================================
+ Hits         2422     2428       +6     
  Misses       3803     3803              

Flag	Coverage Δ
unittests	38.96% <93.75%> (+0.05%)	⬆️

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[RoyStegeman]

Different from what the title of this PR suggests, changes go beyond typehints and the interface is also being changed. In particular, OperatorEvolution now separates the static and time-dependent parts into two attributes. This change is propagated and many of the changes in this PR are to accommodate this change.

Before I continue reviewing. What is the reason for this change?

[RoyStegeman]

in DynamiqsEngine.evolve time is an Iterable. An iterable can also be a dict which is not an ArrayLike. So this typehint is too restrictive (actually it was already before, but it still is). I'm assuming that Iterable is more general than necessary, so in that case you could change that one instead of here.

[RoyStegeman]

qibolab/benchmarks/engine_sweep.py

Line 81 in dd6f942

hamiltonian = 0

This hamiltonian can be 0 or something else, but eventually is also passed to evolve.