[Feat] Add MVMoE model

[RoyalSkye]

Description

This PR adds support for MVMoE in the multi-task VRP setting. In specific, the token-level input-choice gating is implemented. Some minor bugs are fixed as well, such as missing the definition of MTVRPInitEmbedding and MTVRPContext.

• [x] An example will be given in 1-mtvrp.ipynb.

Motivation and Context

A step towards foundation VRP models.

• [ ] I have raised an issue to propose this change (required for new features and bug fixes)

Types of changes

What types of changes does your code introduce? Remove all that do not apply:

• [x] Bug fix (non-breaking change which fixes an issue)
• [x] New feature (non-breaking change which adds core functionality)
• [x] Breaking change (fix or feature that would cause existing functionality to change)
• [ ] Documentation (update in the documentation)
• [x] Example (update in the folder of examples)

Checklist

Go over all the following points, and put an x in all the boxes that apply. If you are unsure about any of these, don't hesitate to ask. We are here to help!

• [ ] My change requires a change to the documentation.
• [x] I have updated the tests accordingly (required for a bug fix or a new feature).
• [ ] I have updated the documentation accordingly.

[RoyalSkye]

I have added an example to 1-mtvrp.ipynb. It seems that the notebook has some issues on my laptop, so I just uploaded a raw version without output 🚀.

[fedebotu]

It seems that the notebook has some issues on my laptop, so I just uploaded a raw version without output

Are you using an M-series Macbook? We had some problems only with MTVRP and only with Apple MPS accelerators :rofl:

[cbhua]

The notebook works smoothly for me! If it has some bug on a Macbook, it's only could be the problem with the M chip 😂 We had experiences on this.

[RoyalSkye]

Maybe this is the reason😂. Could you help me to upload a new version that overwrites the current one? Thanks!

[RoyalSkye]

Thank Chuanbo. Looks great! I have nothing to add to this PR.

[cbhua]

Great job! After @LTluttmann having a look let's launch! 🚀

[LTluttmann]

awesome work! Clean and well documented code, was a pleasure to read it haha! Merging now