refactoring emulator results.py file

[lballerio]

In this small PR we simply anticipate the results.py modification that were first introduced in PR #1401.

Checklist:

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

[codecov]

Codecov Report

❌ Patch coverage is 97.87234% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 37.54%. Comparing base (1df029e) to head (086b06d).
⚠️ Report is 19 commits behind head on main.

Files with missing lines	Patch %	Lines
src/qibolab/_core/instruments/emulator/emulator.py	90.90%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1412      +/-   ##
==========================================
+ Coverage   37.36%   37.54%   +0.18%     
==========================================
  Files         114      114              
  Lines        5736     5755      +19     
==========================================
+ Hits         2143     2161      +18     
- Misses       3593     3594       +1     

Flag	Coverage Δ
unittests	37.54% <97.87%> (+0.18%)	⬆️

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[lballerio]

Hi @alecandido, am I wrong or this np.stack call is superfluous?

np.stack(np.unravel_index(meas, hamiltonian.dims))[i]

and we can simply write:

np.unravel_index(meas, hamiltonian.dims)[i]

[alecandido]

Yes, it looks irrelevant: np.unravel_index() is returning a tuple of arrays, and np.stack() concatenating them into a single one. If you were applying any other indexing operation, it would matter. But if you index over the most external one, the indexing of the array n + 1-dimensional array coincides with the indexing of the tuple of n-dimensional arrays

import numpy as np
a = tuple(np.arange(i*100, i*100 + 10) for i in range(10, 20))
np.stack(a)[3] == a [3]

(for higher dimensions, just .reshape() the result of np.arange())

[alecandido]

Just reading it, it seems alright. If you also tested and are reasonably confident with the results, we could merge.

I only left few marginal comments.

[alecandido]

Actually, since this only depends on the function inputs, better to place it as soon as possible in the function (instead of performing first some computation, to then fail anyhow).

[alecandido]

As suggested before, move this explanation in the module-level docstring. And then just comment locally using the defined symbols