Remove qextract_p and qinst_p from forced-order primitives

[Mandrenkov]

Context:

Generating the JAX jaxpr from a PennyLane QNode composed of many (unrolled) gates can be quite slow. This is partially due to the topological sorting of jaxpr equations which form a dependency graph with a near-quadratic number of edges. Most of these edges are a result of forced-order constraints that are added using:

for i, q in fixedorder:
   # [This nested loop introduces quadratic complexity.]
    for b in boxes[i + 1 :]:
        b.parents.append(q)

Currently, the set of forced-order primitives is {qdevice_p, qextract_p, qinst_p}; however, the data dependencies of qextract_p and qinst_p should already be captured by the relationships of input and output variables in the jaxpr.

Description of the Change:

• Removed qextract_p and qinst_p from the list of forced-order primitives.

Benefits:

• Obtaining the jaxpr of a quantum circuit with many gates is quadratically faster (up to a factor).

Possible Drawbacks:

• Constraints between qextract_p and qinst_p primitives must (continue to) be encoded via input-output variable dependencies.

Related GitHub Issues:

None.

[rmoyard]

Great first contribution to Catalyst 💯

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (cc62f65) 99.56% compared to head (94065f1) 99.56%.

Additional details and impacted files

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##             main     #469   +/-   ##
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  Coverage   99.56%   99.56%           
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  Files          43       43           
  Lines        7666     7666           
  Branches      514      514           
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  Hits         7633     7633           
  Misses         17       17           
  Partials       16       16           

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[erick-xanadu]

This change was driven by a performance issue, correct? What was the impact on performance before and after this change? Just saw the ticket comment. 😎

Thanks!