[Strings3] Update documentation and codebase with new string representations of X, Y, Z and Identity

[Qottmann]

Updating docs and codebase with qml.X/Y/Z/I(*) instead of qml.PauliX/Y/Z(*) and qml.Identity(*) following #5116 and updating composite operation outputs in docs according to #5138

ToDo

• Improve https://docs.pennylane.ai/en/stable/introduction/operations.html#qubit-operators
• Update outputs in docs with PauliX(wires=*)
• Update outputs in docs with SProds and other composites (i.e. with extra spaces and removing unnecessary brackets)
• manually check & update all intro pages
• manually check & update pulse docs
• All tests pass
• changelog

[trbromley]

Thanks @Qottmann! This was from a find-and-replace right? 😆

My main reflection is that, given we are pointing everyone to X, Y, Z, we should probably improve the docs for them (e.g.) - maybe making PauliX the alias instead.

[josh146]

Yep this is actually quite important:

My main reflection is that, given we are pointing everyone to X, Y, Z, we should probably improve the docs for them (e.g.) - maybe making PauliX the alias instead.

We should update the qml.X etc. docstrings to be the main docstrings. We should also have the qml.PauliX have minimal docstrings with a ..seealso:: linking to the X version

[Qottmann]

Excellent point!

A lot of the codebase seems to rely on checking not just isinstance(op, PauliX) but also directly checking the name of the class or instance. I went with the solution to have X/Y/Z/I be the main classes and overwrite the class name. PauliX/Y/Z/Identity are now just aliases. This way the mechanics in the codebase and any plugins remain functioning.

I also tried subclassing but it turned out difficult to have both isinstance(qml.PauliX(0), qml.X) and isinstance(qml.X(0), qml.PauliX). I also tried with a metaclass to overwrite the __instancecheck__ but I couldnt get it to work (happy to discuss though)

[albi3ro]

Well that was painful to get through...

[Qottmann]

Now:

and

Sounds good?