Legacy operator arithmetic deprecation

[mudit2812]

Context: Legacy op math is being deprecated.

Description of the Change:

• qml.ops.Hamiltonian is deprecated.

• qml.operation.Tensor is deprecated.

• qml.operation.disable_new_opmath and qml.operation.enable_new_opmath (as well as the context managers) are deprecated.

• qml.operation.convert_to_legacy_H is deprecated.

• PauliWord.hamiltonian and PauliSentence.hamiltonian is deprecated.

• qml.pauli.simplify is deprecated.

• Removed use_legacy_opmath and use_new_opmath fixtures. Instead, we use either legacy_opmath_only, new_opmath_only, or no fixture, as appropriate for the tests.

• Added warning suppression for all the above deprecation warnings to pytest.ini. Additionally, I also added the same warning suppressions to conftest.py for the case when --disable-opmath=True, since the warning filters in pytest.ini only suppress warnings that come from inside tests, not ones that come from outside the tests, such as when deprecated code is used for creating parameters for a test.

Benefits:

Possible Drawbacks: In some instances, there will be a LOT of deprecation warnings at the same time. The most impactful example of this if if using qml.qchem.molecular_hamiltonian, qml.qchem.qubit_observable. These using simplify and PauliSentence.hamiltonian with legacy op math, so there will be up to 4 warnings at once when using these.

Additionally, qml.qchem.tapering.symmetry_generators and qml.qchem.tapering.clifford use PauliSentence.hamiltonian when legacy op math is enabled, so there will be up to 3 warnings at once when using these.

Related GitHub Issues:

I will run the test suite with disable_new_opmath set to True once before merging to confirm that legacy op math tests do not fail.

[sc-71940] [sc-66727] [sc-66724]

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 99.39%. Comparing base (d9e833a) to head (455c9a9). Report is 4 commits behind head on master.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #6287 +/- ## ========================================== - Coverage 99.70% 99.39% -0.32% ========================================== Files 445 445 Lines 42260 42271 +11 ========================================== - Hits 42137 42016 -121 - Misses 123 255 +132 ```

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[PietropaoloFrisoni]

Should we also replace use_legacy_opmath with legacy_opmath_only in this test?

[mudit2812]

Looks like running some tests only when legacy op math is active has caused some humps with codecov. Not sure if it should be addressed or not.

[PietropaoloFrisoni]

To fix the error in:

tests/optimize/test_momentum_qng.py::TestOptimize::test_single_qubit_vqe_using_expval_h_multiple_input_params (added today) and tests/optimize/test_qng.py::TestOptimize::test_single_qubit_vqe_using_expval_h_multiple_input_params

I guess we can safely remove the recwarn parameter. I am not sure about the other failures

[mudit2812]

It looks like I just need to add back some of the duplicate tests we had for legacy opmath tests to pass. For some reason, the legacy opmath enable only happens after all the parameters for parametrized tests are created, so we're ending up in a situation where we create LinearCombinations and Prods when Hamiltonians and Tensors are expected.

[mudit2812]

Confirmation that all tests pass with --disable-opmath=True here.

[Alex-Preciado]

Proceeding to force merge after discussing with @mudit2812 !! Context: Tests that used to run in CI for legacy opmath have been changed to only run in the Plugin Test Matrix