Update block encoding demo

[soranjh]

Update the demo on block encoding with matrix access oracles to:

1. Add the newly implemented qml.FABLE to the demo.
2. Correct an equation to allow block encoding 2 x 2 matrices.

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• Pull Request ID: 1080
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[soranjh]

Looks good @soranjh! Do you want to refactor this demo cause there are some lines

e.g. wire_order = ancilla_wires + wires_i[::-1] + wires_j[::-1] M = len(A) * qml.matrix(circuit,wire_order=wire_order)().real[0:len(A),0:len(A)]

in the code that don't work quite the same when using the template instead. With the template, we don't need to set the wire_order this way, just need M = len(A) * qml.matrix(circuit)().real[0 : len(A), 0 : len(A)]

There's also code like wire_map = {control_index : wire for control_index, wire in enumerate(wires_j + wires_i)} that can be replaced with what's in the template wire_map = dict(enumerate(wires_j + wires_i))

Not a super big deal and the demo works as is though.

Thanks @austingmhuang, the demo codes are a bit verbose for educational purposes, so no need to change them if they run correctly.

[soranjh]

• We could optionally show a circuit using qml.Fable and then draw the circuit using my_circuit.tape.expand().draw() to show the decomposed operation

Checked and confirmed locally but adding it here breaks the current nice workflow.

• Just make sure that the link to the Fable template works well once the CI builds

Confirmed with @KetpuntoG that it will work when merged.