Support drawing qml.cond: Part 1

[mudit2812]

Context:
Adding support for qml.cond for mid-circuit measurement drawing with qml.draw. This PR adds support for using one mid-circuit measurement with multiple qml.conds, where each qml.cond uses the same mid-circuit measurement. An example of the drawing is shown below:

import pennylane as qml

def circuit():
    qml.RX(0.5, 0)
    qml.RX(0.5, 1)
    m0 = qml.measure(0, reset=True, postselect=1)
    qml.cond(m0, qml.MultiControlledX)(wires=[1, 2, 3, 0], control_values="110")

    qml.CNOT([3, 2])
    qml.cond(m0, qml.ctrl(qml.MultiRZ, control=[1, 2], control_values=[True, False]))(0.5, wires=[0, 3])

    return qml.expval(qml.PauliZ(0))

print(qml.draw(circuit)())

0: ──RX(0.50)──┤↗₁│  │0⟩─╭X────╭MultiRZ(0.50)─┤  <Z>
1: ──RX(0.50)───║────────├●────├●─────────────┤     
2: ─────────────║────────├●─╭X─├○─────────────┤     
3: ─────────────║────────╰○─╰●─╰MultiRZ(0.50)─┤     
                ╚═════════╩═════╝                   

Note: This PR contains work that is foundational for follow up PRs but isn't used for drawing the cases mentioned above. I'll add arbitrary testing to get full coverage, but the aforementioned cases will be the only ones considered to currently be complete.

Description of the Change:

drawable_layers:

• drawable_layers no longer stacks mid-circuit measurements used by a qml.cond in the same layer.
• drawable layers now considers all wires under a mid-circuit measurement used for conditioning to be used.
• drawable_layers now considers all wires under a qml.cond to be used.

tape_text:

• Added helper functions to tape_text.py for correctly adding labels for mid-circuit measurements and qml.conds, as well as for correctly padding the width of the layers for unused wires.
• Added helper function to tape_text.py for getting information about connectivity between mid-circuit measurements and conditional operations to keep track of when to start and stop drawing classical bits.
• tape_text() now tracks a bit_map, which stores a mapping between mid-circuit measurements and classical bits that are drawn for them. Classical bits are drawn using double lines ═, and are placed under the wires in the drawing. Each mid-circuit measurement used for conditioning uses a classical bit. Measurements not used for conditioning are not allocated a bit.
• tape_text() now pads the classical bits between each layer of the drawing along with the wires.
• This PR also addresses [BUG] Drawing circuits with arbitrary measurements fails #4809. The changes pertaining to that are contained in tape_text.py::_add_measurements().

Other changes:

• Added label to qml.transforms.Conditional.
• Adjusted qml.equal to consider postselection when comparing MidMeasureMPs. This was missed in Allow postselecting on mid-circuit measurements #4604.

Benefits:
Drawing support for mid-circuit measurements is better :)

Possible Drawbacks:
Code in tape_text.py is less readable. After a discussion with @trbromley , we decided that the best course of action is to leave refactoring as technical debt for the end of the release cycle. Reviews pertaining to major refactors are still appreciated :)

Related GitHub Issues:
#4809

[mudit2812]

[sc-49112]

[mudit2812]

[sc-49764]

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (370d9d4) 99.65% compared to head (f1a9df1) 99.64%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4803      +/-   ##
==========================================
- Coverage   99.65%   99.64%   -0.02%     
==========================================
  Files         385      385              
  Lines       34719    34569     -150     
==========================================
- Hits        34599    34446     -153     
- Misses        120      123       +3     

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[albi3ro]

This is looking really and an impressive amount of work 🎉

[timmysilv]

amazing! some of the content can be a bit tough to parse through... but that's tape_text for ya