Selecting wires of default.tensor at runtime if they are not provided by user

[PietropaoloFrisoni]

Context: In the default.tensor device, we need to create an initial state (e.g. in the MPS case using the MPS_computational_state function) in quimb, which requires the number of qubits to be defined.

We have a very cryptic error from quimb as soon as we compute something with this device if there is a mismatch between the circuit wires and those on the device.

To circumvent this issue, we previously made wires the last positional argument for this device (as in lightning.qubit), checking at runtime if there's a mismatch between the wires on the device and those associated with the circuit.

Description of the Change: We now instantiate the quimb tensor directly in the simulate function. The wires on the device are selected at runtime to match those on the circuit in case they haven't been provided by the user.

Otherwise, default.tensor raises an informative error if the wires have been provided by the user and there is a mismatch.
This allows the wires argument to be optional for this device.

Benefits: This makes the default.tensor more flexible and, generally, more compatible with the existing devices in PL.

Possible Drawbacks: As stated above, we have an error if there is a clear mismatch, such as:

dev = qml.device("default.tensor", wires=2)
ops = [
    qml.Identity(wires=0),
    qml.Identity(wires=(0, 1)),
    qml.CNOT(wires=[0, 1]),
    qml.RX(phi=0.5, wires=0),
    qml.RY(phi=1, wires=5),
    qml.RX(phi=2, wires=1),
]
measurements = [qml.expval(qml.PauliZ(wires=15))]
tape = qml.tape.QuantumScript(ops, measurements)

>>> dev.execute(tape)
WireError: Mismatch between circuit and device wires. Circuit has wires [0, 1, 5, 15]. Tensor on device has wires [0, 1]

which seems justified. However, the error disappears if we don't provide wires:

dev = qml.device("default.tensor")
ops = [
    qml.Identity(wires=0),
    qml.Identity(wires=(0, 1)),
    qml.CNOT(wires=[0, 1]),
    qml.RX(phi=0.5, wires=0),
    qml.RY(phi=1, wires=5),
    qml.RX(phi=2, wires=1),
]
measurements = [qml.expval(qml.PauliZ(wires=15))]
tape = qml.tape.QuantumScript(ops, measurements)

dev.execute(tape)
1.0

Related GitHub Issues: None.

Related Shortcut Stories: [sc-64007]

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.66%. Comparing base (2a36a35) to head (86e30db).
Report is 258 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #5744      +/-   ##
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- Coverage   99.67%   99.66%   -0.01%     
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  Files         413      413              
  Lines       39254    38957     -297     
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- Hits        39126    38828     -298     
- Misses        128      129       +1     

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[multiphaseCFD]

Thanks @PietropaoloFrisoni ! Just a few a comments and questions.

[mudit2812]

Thanks Pietro!

[multiphaseCFD]

Thanks @PietropaoloFrisoni !