Add function for convert circuit parameters to numpy

[albi3ro]

Currently, we use the qml.tape.Unwrap context manager to convert parameters to numpy before sending them to a device.

This context manager modifies the input in-place and resets it at the end. It uses QuantumScript.set_parameters, which iterates over all operations, modifies them in place, and then checks batching.

Notably, it calls op._check_batching for every single parameter that gets set, not every operator that has parameter set.

This causes massive performance problems for large observables. We are also setting the parameters for operators that might not even need it.

This PR proposes a transform convert_to_numpy_parameters that creates a new quantum script with numpy or built-ins parameters. Currently, it is in the devices.preprocessing module, as it was designed thinking about new devices, and making it easy for new devices to convert to vanilla numpy parameters.

Given the huge performance problems with Unwrap, we should consider using this everywhere instead.

When using a Sum object instead of a Hamiltonian, Unwrap takes 1min 36seconds, a great deal more time. convert_to_numpy_parameters takes 8.42 ms, twice the amount of time as for Hamiltonian, but still substantially faster than the Unwrap case.

If we are going to immediately replace Unwrap with this instead, we should better consider the location for this conversion utility.

[codecov]

Codecov Report

Merging #3899 (54532f5) into master (e5f0476) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #3899   +/-   ##
=======================================
  Coverage   99.56%   99.56%           
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  Files         348      349    +1     
  Lines       30382    30410   +28     
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+ Hits        30250    30278   +28     
  Misses        132      132           

Impacted Files	Coverage Δ
pennylane/transforms/__init__.py	100.00% <100.00%> (ø)
...ennylane/transforms/convert_to_numpy_parameters.py	100.00% <100.00%> (ø)

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[timmysilv]

it's as if you magically looked at my local branch and nabbed the changes I made! looks excellent. I want to confirm a thing or two before approving, but I will likely approve today.

[mudit2812]

Thanks Christina! I think that this looks good. Is there a timeline for when this would replace tape.unwrap?

[albi3ro]

[sc-36752]

[albi3ro]

Thanks Christina! I think that this looks good. Is there a timeline for when this would replace tape.unwrap?

@mudit2812 We are working on a PR to replace the use of unwrap in the interfaces module.