Multiple Datasets - Update diagnostics

[JPXKQX]

Update all diagnostic callbacks (e.g., PlotSample, etc.) to support the new dictionary-based model output format.

Goal

Train a model that returns a dictionary-like output, with all diagnostic options enabled.

Scope

• Config: Update the configuration to include arguments specifying the dataset(s) to be used for plotting.
• Callbacks: This PR primarily updates the plotting-related callbacks used during training.

Notes

• This can be started in parallel after the base multi-dataset output structure from Multiple Datasets I - Base support in anemoi-core #230 is in place.