Allow for multiple input datasets in training

[b8raoult]

Is your feature request related to a problem? Please describe.

No response

Describe the solution you'd like

Each dataset can have its own resolution, variables, etc.

The need is the same for downscaling, and will be the same for observations. We need a data loader (or several) that reads from different training-datasets and use a different set indices and different set of normalisations and imputers. The two inputs will be then merged later (e.g. in the encoder).

Describe alternatives you've considered

No response

Additional context

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Organisation

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