feat: flexible forecaster [experimental]

[dietervdb-meteo]

Description

This is a draft PR that adds to inference support for the flexible forecaster ecmwf/anemoi-core#1070

This code serves two purposes: 1. be able to run experimental inference with a flexible forecaster. 2. highlight some code changes the would be required in inference if we generalize the forecaster.

Note that this is a draft PR. It might very well be the case that this code will not [from this branch, or as is] in the end be merged to main. Still I set up this PR to provide some visibility and to allow for easy comparison the changes introduced wrt main.

Example config to use the new runner:

runner:
  flexible:
    input_offsets: ["-6H", "0H"]
    output_offsets: ["1H", "2H", "3H", "4H", "5H", "6H"]
    step_shift: "6H"

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