Allow wire_options per wire MPL drawers

[dwierichs]

Feature details

On the level of mid-scale algorithms, it might be nice to differentiate between different "types" of wires, for example by coloring them differently, or giving them distinct line styles. As an example, in this demo on preparing matrix product states, there are auxiliary bond qubits and physical qubits, and coloring them differently would be a neat thing to do.

Implementation

This should be as simply as allowing a nested dictionary for wire_options with wires as keys and inner wire_options dictionaries. If such a nested dictionary is passed, we only need to iterate over it, instead of reusing the same for all wires.
To make the idea clear, here are some example inputs that already are supported, and some that should work after adding this feature:

@qml.qnode(qml.device("default.qubit"))
def node(x):
    qml.RX(x, 0)
    qml.Hadamard(1)
    qml.CNOT([0, 1])
    return qml.expval(Z(0) @ Y(1))

# Default drawing:
qml.draw_mpl(node)(0.52)
# Currently already supported: Change wire options for all wires:
wire_options = {"color": "orange"}
qml.draw_mpl(node, wire_options=wire_options)(0.52) # All wires are orange
# New feature: Only color the first wire orange
wire_options = {0: {"color": "orange"}}
# New feature: Only make the second wire bold
wire_options = {1: {"linewidth": 5}}
# New feature: Make the first wire dashed and the second teal
wire_options = {0: {"linestyle": "--"}, 1: {"color": "teal"}}

For a circuit with many wires, it would be annoying to have to pass information for all wires. That's why the following should also be supported:
Specify the wire_options for all wires at the same time, but allow single wire entries to be in that dictionary, refining the options for a given wire:

@qml.qnode(qml.device("default.qubit"))
def node(x):
    for w in range(10):
         qml.Hadamard(w) 
    return qml.expval(Z(0) @ Y(1))

# New feature: Make all wires blue and bold, except for wires 2 and 6, which are dashed and another color
wire_options = {"color": "blue", "linewidth": 5, 2: {"linestyle": "--", "color": "red"}, 2: {"linestyle": "--", "color": "orange"}}
qml.draw_mpl(node, wire_options=wire_options)(0.52)

Suggestion for an updated description in the docstrings:

wire_options (dict): matplotlib formatting options for the wire lines. In addition to options
    passed to matplotlib, this ``dict`` may contain entries with wire labels as keys   
    and ``dict``\ s as values, which contain line formatting options for the specific wire.

How important would you say this feature is?

1: Not important. Would be nice to have.

Additional information

This is really just a small cosmetic idea.

[zazabap]

Dear @dwierichs , I am new to the community and hope to find some good first issue to contribute. If this issue is not yet taken by other people, could I possibly work on it and later submit a PR? I worked a lot on MPS and hope to improve a bit on this part. Maybe later adding similar features to MERA related implementations.

[dwierichs]

Hi @zazabap
Thanks for your interest in contributing, this is great! 🎉
I think this is a neat small extension that definitely could be a good first issue to work on.
As there is a small UI decision involved, namely how to realize the individual options for different wires, let me discuss this internally first :) I'll get back to you soon!

[dwierichs]

@zazabap I discussed internally, and I added details to the issue description of how the user interface (UI) should look for this feature. Would you like to work on it? :) Let me know in case you have any questions!

[zazabap]

Thanks a lot for the internal discussion! I will start a pull request and work on it. The rest of the fix and new features will attribute to #6486

[albi3ro]

Closed by #6486