fixing cupy tests failing

[scarrazza]

Closes #1072 and fixes further cupy failures in the quantum_info module.

Checklist:

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

[scarrazza]

@renatomello, I have applied some changes to the diamond_norm because when running all tests together the global backend is changed and does not match qibojit-cupy, therefore the call to to_numpy does not work as expected and we get this:

FAILED tests/test_quantum_info_metrics.py::test_diamond_norm[qibojit-cupy-1] - TypeError: Implicit conversion to a NumPy array is not allowed. Please use `.get()` to construct a NumPy array explicitly.
FAILED tests/test_quantum_info_metrics.py::test_diamond_norm[qibojit-cupy-2] - TypeError: Implicit conversion to a NumPy array is not allowed. Please use `.get()` to construct a NumPy array explicitly.

[scarrazza]

@stavros11, I have fixed the issue reported in #1072 by reducing the memory required by the specific test. We had a similar problem back in 2022: we are allocating "many" large objects in the GPU memory and pytest does not give us access to a proper garbage collector, so the cupy driver fails deallocating memory. By reducing the memory the deallocation is faster and the pytest loop (allocation/deallocation) works.

I have also fixed a missing casting in samples_to_decimal, supposing that we want to keep the operation on CPU, otherwise pytest will raise:

FAILED tests/test_measurements.py::test_circuit_copy_with_measurements[qibojit-cupy-None] - TypeError: Argument 'array' has incorrect type (expected cupy._core.core._ndarray_base, got numpy.ndarray)
FAILED tests/test_measurements.py::test_circuit_copy_with_measurements[qibojit-cupy-accelerators1] - TypeError: Argument 'array' has incorrect type (expected cupy._core.core._ndarray_base, got numpy.ndarray)
FAILED tests/test_measurements.py::test_circuit_copy_with_measurements[qibojit-cupy-accelerators2] - TypeError: Argument 'array' has incorrect type (expected cupy._core.core._ndarray_base, got numpy.ndarray)
FAILED tests/test_measurements.py::test_circuit_copy_with_measurements[qibojit-cupy-accelerators3] - TypeError: Argument 'array' has incorrect type (expected cupy._core.core._ndarray_base, got numpy.ndarray)

Please let me know if you agree.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (2f89525) 100.00% compared to head (19adef4) 100.00%.
Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff            @@
##            master     #1147   +/-   ##
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  Coverage   100.00%   100.00%           
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  Files           67        67           
  Lines         9589      9588    -1     
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- Hits          9589      9588    -1     

Flag	Coverage Δ
unittests	100.00% <100.00%> (ø)

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[stavros11]

Thank you for the fixes. Tests are now passing on my computer and the cluster.