Support drawing mid-circuit measurement statistics with qml.draw

[mudit2812]

Context:
This PR adds support for drawing the collection of mid-circuit measurement statistics with qml.draw. An example is shown below:

def circ():
    qml.Hadamard(0)
    m0 = qml.measure(0)
    qml.Hadamard(1)
    m1 = qml.measure(1)
    qml.Hadamard(2)
    m2 = qml.measure(2)
    qml.Hadamard(3)
    qml.measure(3)
    qml.cond(m0, qml.PauliX)(0)
    qml.cond(m0 & m1, qml.PauliY)(1)
    return qml.expval(m2), qml.sample([m1, m2])

print(qml.draw(circ)())

0: ──H──┤↗├──────────────────────────X────┤                    
1: ──────║───H──┤↗├──────────────────║──Y─┤                    
2: ──────║───────║───H──┤↗├──────────║──║─┤                    
3: ──────║───────║───────║───H──┤↗├──║──║─┤                    
         ╚═══════║═══════║═══════════╩══╣                      
                 ╚═══════║══════════════╩═╡        ╭Sample[MCM]
                         ╚════════════════╡  <MCM> ╰Sample[MCM]

Description of the Change:

• Refactor tape_text to only run through one loop containing all layers, not separate loops for op layers and measurement layers. The logic inside the loop has been adjusted accordingly. I also added big block comments to make the separation between different steps more obvious.
• Add helpers to tape_text.py to add grouping symbols and labels for mid-circuit measurement statistics correctly.
• Update drawable_layers to find layers for mid-circuit measurement statistics correctly. This included adding a new helper to recursively find layers for terminal measurements that use measurement values.
• The bit map is now initialized with None values before everything else, and this is used to keep track of which mid-circuit measurements are used. Mostly a refactor to tidy up the code.
• Remove code from draw_mpl that deferred measurements unconditionally for qnodes with mid-circuit measurements. This code was removed from qml.draw in Draw qml.cond: Part 2; qml.map_wires correctly maps nested tape wires #4901 and qml.draw_mpl is now updated as well.
• The example in the iterative_qpe docstring was written before we had advanced mid-circuit measurement support and drawing capabilities, so I updated the example as well.

Benefits:

• We can draw mid-circuit measurement statistics!
• Code related to the text drawer is slightly more readable.

Possible Drawbacks:

Related GitHub Issues:

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (5eb005e) 99.49% compared to head (2e2ae25) 99.49%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4930      +/-   ##
==========================================
- Coverage   99.49%   99.49%   -0.01%     
==========================================
  Files         388      388              
  Lines       35467    35245     -222     
==========================================
- Hits        35289    35066     -223     
- Misses        178      179       +1     

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[mudit2812]

[sc-49830]

[timmysilv]

nice stuff!

[albi3ro]

👍 Looks great :)

[timmysilv]

✏️ 🔬 🎉