Allow qml.for_loop and qml.while_loop to fallback to the Python interpreter if a compiler is not available.

[josh146]

Context:

In Catalyst, catalyst.for_loop and catalyst.while_loop will seamlessly fallback to Python interpretation when @qjit is not present. However, qml.for_loop and qml.while_loop will instead raise an error when @qjit is not present, which is not ideal.

Description of the Change:

When @qjit is not present, qml.for_loop and qml.while_loop will now simply execute the for/while loop in standard Python.

Benefits:

• This will allow the same code to work with and without @qjit, without needing to modify the workflow.

• This will also allow templates and operation decompositions to use qml.for_loop and qml.while_loop internally, with no changes for standard Python execution, but with loops captured automatically when using @qjit.

Possible Drawbacks:

• Not a drawback per se, more of a note, but the ForLoopCallable and WhileLoopCallable classes introduced in this PR might be needed (and extended) to support capturing for loops/if statements as JAXPR primitives(?). They are based on the equivalent classes from Catalyst.

Related GitHub Issues: n/a

[github-actions]

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Please edit doc/releases/changelog-dev.md with:

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.65%. Comparing base (db7b281) to head (dc4e0f9).
Report is 295 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #6014      +/-   ##
==========================================
- Coverage   99.66%   99.65%   -0.01%     
==========================================
  Files         427      427              
  Lines       41119    40855     -264     
==========================================
- Hits        40980    40714     -266     
- Misses        139      141       +2     

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[josh146]

[sc-69431]

[josh146]

[sc-69432]

[dime10]

Great addition, no comments from my side :)

[albi3ro]

Nothing blocking, but just a few nitpicks.

My main question is about where the source code for this should live long term. I'm happy to leave these here for now, but we should consider moving it to either the capture module or somewhere new.

[josh146]

My main question is about where the source code for this should live long term. I'm happy to leave these here for now, but we should consider moving it to either the capture module or somewhere new.

Yeah I agree, qjit_api.py is not the right place for it.

[albi3ro]

🎉