Resolve error from texted-based circuit drawer for tapes

[dime10]

Context:
As discovered in issue #1967, the text-based circuit drawer throws an error when the operations list of a circuit contains a tape. This can happen manually when nesting a tape inside a circuit or tape, e.g.:

import pennylane as qml

dev = qml.device("default.qubit", wires=3)

@qml.qnode(dev)
def qpe_circuit():
    with qml.tape.QuantumTape():
        qml.PauliX(0)
        qml.CNOT(wires=[0,2])
    qml.Hadamard(1)
    return qml.state()

print(qml.draw(qpe_circuit)())

but also when an operation needs to be decomposed before printing, such as an adjoint template without a defined .adjoint() method that manipulates the .inv attribute instead, e.g. (v0.19.0 only, fixed in current dev branch):

dev = qml.device("default.qubit", wires=2)

@qml.qnode(dev)
def qpe_circuit():
    qml.adjoint(qml.QuantumPhaseEstimation)(
        qml.PauliX.matrix,
        target_wires=[0],
        estimation_wires=[1],
    )
    return qml.state()

print(qml.draw(qpe_circuit)())

Description of the Change:
The issue is resolved by adding support for drawing tapes as operations to the circuit drawer.
The contents of the tape are also drawn separately and attached to the main circuit drawing in a manner similar to how matrix arguments are already included.
Nested tapes are also supported and rendered by recursive traversal, e.g.:

@qml.qnode(dev)
def qpe_circuit():
    with qml.tape.QuantumTape():
        qml.PauliX(0)
        qml.CNOT(wires=[0,2])
        with qml.tape.QuantumTape():
            qml.QuantumPhaseEstimation(qml.PauliY.matrix, target_wires=[1], estimation_wires=[2])
            qml.CNOT(wires=[1,2])
    qml.Hadamard(1)
    with qml.tape.QuantumTape():
        qml.SWAP(wires=[0,1])
    return qml.state()

is drawn as:

 0: ──╭QuantumTape:T0─────╭QuantumTape:T1──╭┤ State 
 1: ──├QuantumTape:T0──H──╰QuantumTape:T1──├┤ State 
 2: ──╰QuantumTape:T0──────────────────────╰┤ State 
T0 =
 0: ──X──╭C───────────────────┤  
 2: ─────╰X──╭QuantumTape:T2──┤  
 1: ─────────╰QuantumTape:T2──┤  
T2 =
 1: ──╭QuantumPhaseEstimation(M0)──╭C──┤  
 2: ──╰QuantumPhaseEstimation(M0)──╰X──┤  
M0 =
[[ 0.+0.j -0.-1.j]
 [ 0.+1.j  0.+0.j]]


T1 =
 0: ──╭SWAP──┤  
 1: ──╰SWAP──┤  

Benefits: No errors are thrown when the drawer attempts to render a tape.

Possible Drawbacks:

Related GitHub Issues: Closes #1967

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[antalszava]

Looks good to me! 🎉 Thanks for resolving this @dime10 🌟 😍

Had a couple of suggestions, but nothing blocking. 👍

[antalszava]

A docstring entry is missing for this argument.

[dime10]

I was wondering if we exclude it from the public API somehow as it's only used internally but I'll add a docstring :)

[antalszava]

Minor suggestion: may be good to rename this argument to make it more descriptive: e.g., custom_offsets or offset_per_unit or something better 😄 .

[dime10]

How about label_offsets?

[antalszava]

How come we'd need a generator solution, would this line correspond to something like this?

charset_string = self.charset if self.charset in CHARSETS else None

[dime10]

Ah originally I was using tape.draw() which only accepts a string descriptor of the charset (self.charset stores the charset class), but since I'm using CircuitDrawer directly now this can be removed. The generator construct is just one way to reverse a dictionary mapping (CHARSETS is a dictionary of {string: class}).

[dime10]

Thanks a lot for your review @antalszava :)

[dime10]

[sc-11982]