refactor(graphs,icon): refactor icon graph node/edge builders

[JPXKQX]

Description

Instead of using ICONNodes, decouple the different parts to be computed in each node and edge builder.

nodes:
  # Data nodes
  data:
    node_builder:
      _target_: anemoi.graphs.nodes.ICONCellGridNodes
      max_level: 3
      grid_filename: icon_grid_0026_R03B07_subsetAICON.nc
  # Hidden nodes
  hidden:
    node_builder:
      _target_: anemoi.graphs.nodes.ICONMultimeshNodes
      max_level: 3
      grid_filename: icon_grid_0026_R03B07_subsetAICON.nc
edges:
  # Processor configuration
  - source_name: hidden
    target_name: hidden
    edge_builders:
      - _target_: anemoi.graphs.edges.ICONTopologicalProcessorEdges
  - source_name: data
    target_name: hidden
    edge_builders:
      - _target_: anemoi.graphs.edges.ICONTopologicalEncoderEdges
  - source_name: hidden
    target_name: data
    edge_builders:
      - _target_: anemoi.graphs.edges.ICONTopologicalDecoderEdges

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📚 Documentation preview 📚: https://anemoi-training--627.org.readthedocs.build/en/627/

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📚 Documentation preview 📚: https://anemoi-graphs--627.org.readthedocs.build/en/627/

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📚 Documentation preview 📚: https://anemoi-models--627.org.readthedocs.build/en/627/

[fprill]

Thanks! Cleaning up and refactoring is always nice!
A few questions:

• Setting for grid_filename in both, data and hidden: Don't you think that this could allow to use inconsistent ICON meshes for the encoder/decoder and the hidden mesh? The edge builder would fail then.
• Minor things: "ICONMultiMeshNodes": Why did you decide to use this spelling? GraphCast writes this as "multi-mesh"... Yes, I know that the previous implementation wasn't consistent :)
• nodeset: NodeSet # set of ICON cell circumcenters - Why does the PR remove this comment? Just a matter of taste, but I also think that a comment like "duplicate edges (bi-directional)" is helpful for some readers....
• Why was the .to(device) necessary (has been added in various places)?
• A question: Is there any danger that the BaseICONNodeBuilder will ever be initialized after the EdgeBuilder?
• Again, thanks for cleaning up. But, regarding the GeneralGraph and the BipartiteGraph: I actually liked these abstract base classes which separate the "ICON stuff" from some general "graph stuff". But I would also agree with your opinion if you have an argument against this mini-hierarchy.