Add frames in lambeq

[dimkart]

Description

The purpose of this task is to add a Frame class to lambeq.backend ^1, representing a higher-order box that contains diagrams inside it:

A core implementation must include a lambeq.backend.grammar.Frame class that can be used in a grammar diagram. It should also have the following features:

• the ability to hold multiple diagrams (like in the image), including diagrams that themselves contain frames
• work with functors
• be drawable, in the sense that Diagram.draw() method is able to render all frames properly, as in the figure above.

Notes

• It may be helpful to refer to the implementation of frames in DisCoPy [^2].

References

[^2]: Vertical frames, horizontal frames, extension

[toumix]

Here's the PR where the picture was taken: https://github.com/discopy/discopy/pull/280 It's close to completion but the reviewing will take a bit of time.