Feature/hierarchical graphs

[icedoom888]

An implementation of hierarchical graph model.
This allows for the creation of a model capable of dealing with multi-layered hierarchical graphs.
The model is agnostic to the choosen architecture (GNN, GraphTransformer).

[FussyDuck]

All committers have signed the CLA.

[mchantry]

@icedoom888 thanks for the contribution.
Could you rebase it on top of
https://github.com/ecmwf/anemoi-models/pull/45/files which should be merged in the next day.
to solve the instantiate issue?

[icedoom888]

Hello @mchantry, @floriankrb, I am currently blocked due to the switch to anemoi-training from aifs-mono.
Will resume working on this and address your comments once my pipeline is functional again!