Distribute PLAID utils requiring complex dependancies

[casenave]

In PR #19 , we want to distribute a pyg loader reading plaid datasets. However, it requires torch and pyg dependencies, which are large, complex: we don't want to add them as dependencies of plaid. See: https://chatgpt.com/share/683d9e1e-06e4-800f-99b6-a1238e9812d3

[casenave]

We decided to developp two additional libs:

• plaid-ops: standardized operation of plaid dataset
• plaid-bridges: conversion to ML framework (with heavy dependencies)