Fix Torch tensor locality with autoray-registered coerce method

[mlxd]

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---

Context: When Torch has a GPU backed data-buffer, failures can occur when attempting to make autoray-dispatched calls to Torch method with paired CPU data. In this case, for probabilities on the GPU, and eigenvalues on the host (read from the observables), failures appeared with qml.dot, and can be reproduced from:

import pennylane as qml
import torch
import numpy as np

torch_device="cuda"
dev = qml.device("default.qubit.torch", wires=2, torch_device=torch_device)
ham = qml.Hamiltonian(torch.tensor([0.1, 0.2], requires_grad=True), [qml.PauliX(0), qml.PauliZ(1)])

@qml.qnode(dev, diff_method="backprop", interface="torch")
def circuit():
    qml.RX(np.zeros(5), 0)  # Broadcast the state by applying a broadcasted identity
    return qml.expval(ham)

res = circuit()
assert qml.math.allclose(res, 0.2)

This pair modifies the registered coerce method for Torch to always automigrate mixed CPU-GPU data to always favour the associated GPU. In addition, this method now also catches multi-GPU data, where tensors do not reside on the same index, and will fail outright. As a longer term solution, moving the Torch GPU dispatch calls to earlier in the stack would be more sound, but this fixes the aforementioned issue, at the expense of always migrating from CPU to GPU.

Description of the Change: As above.

Benefits: Allows automatic data migration from host to device when using a GPU backed tensor. In addition, will catch multi-GPU tensor data when using Torch, and fail due to non-local representations.

Possible Drawbacks: Auto migration may not always be wanted. The alternative solution is to always be explicit about locality, and move the eigenvalue data to exist on the device at a higher layer in the stack.

Related GitHub Issues: #5269 introduced changes that resulted in GPU errors.

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[mlxd]

[sc-59860]

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.67%. Comparing base (30f69b0) to head (c648974).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #5438      +/-   ##
==========================================
- Coverage   99.68%   99.67%   -0.01%     
==========================================
  Files         402      402              
  Lines       37527    37246     -281     
==========================================
- Hits        37407    37125     -282     
- Misses        120      121       +1     

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[Qottmann]

Thanks again so much @mlxd !

Should we add a little changelog entry for this improvement/bug fix? Else looks good with everything green 👍

[mlxd]

Thanks @Qottmann and @mudit2812. Will update the changelog and push this through

[lillian542]

Looks good to me!