Add qml.to_openqasm transform

[SimoneGasperini]

This PR adds a transform to make the conversion of a quantum circuit into the corresponding OpenQASM 2.0 program easier and more user-friendly. The qml.to_openqasm function is implemented in a new openqasm module under the qml.transforms subpackage. Not sure this is the right location in the Pennylane repository.

A quite extensive set of unit and integration tests for the QuantumScript.to_openqasm method is already available in test_qasm.py. I will add a detailed description of what the transform is doing in the docstring, including a simple usage example.

[sc-83360]

[albi3ro]

Just thinking here: would it make sense it to live in the io module? We already have the load_qasm function there. So it might make sense.

[SimoneGasperini]

Just thinking here: would it make sense it to live in the io module? We already have the load_qasm function there. So it might make sense.

Ok thank you Christina! I move it to the io module then.

[PietropaoloFrisoni]

Just thinking here: would it make sense it to live in the io module? We already have the load_qasm function there. So it might make sense.

@albi3ro I think this is a good idea! Since this is not really a 'transform' but more a new public function, it would probably be confusing to put it in the pennylane.transforms module

[SimoneGasperini]

I added some unit tests checking the OpenQASM 2.0 code for a few different circuits but actually most of the work done by the transform already has an extensive set of tests for the QuantumScript.to_openqasmmethod (see here) so I'm not sure this is the right way to go in this case. Any kind of suggestion would be appreciated ;)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.69%. Comparing base (e01b47c) to head (1978259).
⚠️ Report is 401 commits behind head on master.

Additional details and impacted files

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##           master    #7393   +/-   ##
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  Coverage   99.69%   99.69%           
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  Files         529      529           
  Lines       50698    50709   +11     
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+ Hits        50543    50554   +11     
  Misses        155      155           

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[astralcai]

LGTM. Congratulations on your first contribution to PennyLane! 🎸

[isaacdevlugt]

Nice @SimoneGasperini! I left some comments on the documentation :)

[albi3ro]

Potentially also include a test for precision?

[PietropaoloFrisoni]

There is already this test for the precision argument, but since we repeated/duplicated tests for rotations and measure_all, we might add one more for consistency at this point