[feature] support post-processors

[JPXKQX]

This PR allows the user to create their own post-processors for the graph. In particular, we provide an implementation of RemoveUnconnectedNodes which may be useful for LAM, where we want to remove the nodes far from the area of interest.

The post-processors are included in the recipe as follows:

nodes:
   ...
edges:
   ...

post_processors:
  - _target_: anemoi.graphs.processors.RemoveUnconnectedNodes
    nodes_name: data
    ignore: cutout_mask # optional
    save_mask_indices_to_attr: indices_connected_nodes # optional

Here, we have 2 optional arguments:

• ignore: It allows the user to ignore some nodes based on an existing attribute. For example, do not drop nodes from inside the limited area, even if they are not connected.
• save_mask_indices_to_attr: It allows the user to store the indices to mask the nodes from the previous graph to the processed graph as an attribute. This may be needed for training/inference using only the points of interest.