Bugfix: Lightning simulators do not support rotations anymore.

[vincentmr]

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---

Context:
Lightning simulators have never or currently do not support the optional rotations argument in their apply method. Sampling measurements are therefore performed in the wrong basis, among other issues.

Description of the Change:
Apply the diagonalizing gates within the sampling measurement section if the device is a Lightning device.
Apply the adjoint of the diagonalizing gates to restore the state for further processing.

Benefits:

Possible Drawbacks:

Related GitHub Issues:

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

❗ No coverage uploaded for pull request base (v0.35.1-rc@47b0394). Click here to learn what that means.

Additional details and impacted files

@@              Coverage Diff              @@
##             v0.35.1-rc    #5343   +/-   ##
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  Coverage              ?   99.64%           
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  Files                 ?      399           
  Lines                 ?    36628           
  Branches              ?        0           
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  Hits                  ?    36497           
  Misses                ?      131           
  Partials              ?        0           

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[Mandrenkov]

I'm with @trbromley in that having at least one test to show this works would be nice!

[vincentmr]

I'm with @trbromley in that having at least one test to show this works would be nice!

I added a test that should pass and provide coverage.

[Mandrenkov]

I added a test that should pass and provide coverage.

Hey @vincentmr, I copied this test to a local version of the RC branch and it passed without these changes:

$ python -m pytest tests/devices/test_lightning_qubit.py -k "test_finite_shots"
=========================================================================================================== test session starts ============================================================================================================
platform darwin -- Python 3.9.13, pytest-8.0.2, pluggy-1.4.0
benchmark: 4.0.0 (defaults: timer=time.perf_counter disable_gc=False min_rounds=5 min_time=0.000005 max_time=1.0 calibration_precision=10 warmup=False warmup_iterations=100000)
rootdir: /Users/mikhail/sc/58357/pennylane/tests
configfile: pytest.ini
plugins: flaky-3.7.0, cov-4.1.0, mock-3.12.0, xdist-3.5.0, benchmark-4.0.0, forked-1.6.0
collected 18 items / 16 deselected / 2 selected

tests/devices/test_lightning_qubit.py ..                                                                                                                                                                                             [100%]
===Flaky Test Report===

test_finite_shots passed 1 out of the required 1 times. Success!

===End Flaky Test Report===

===================================================================================================== 2 passed, 16 deselected in 0.10s =====================================================================================================

Are you sure we're testing the right thing here?

Edit: This was done using Lightning v0.35.0.

[vincentmr]

Are you sure we're testing the right thing here?

Yes and no. Like I mentioned here, I wrote a test that passes with 0.35 and covers the lines, making sure nothing awfully wrong is going on. But the exhaustive tests and related fixes are going to be in the Lightning repo.