Issue #, if available: #4

Description of changes:

• This PR adds multivariate forecasting support to fev.Task. There are only two changes required to define multivariate tasks:
  1. The user must set target_column to a list[str].
  2. Predictions must be formatted as a DatasetDict where each key corresponds to the name of the target column and each value is a Dataset containing the predictions for this column in format compatible with task.predictions_schema.

By submitting this pull request, I confirm that you can use, modify, copy, and redistribute this contribution, under the terms of your choice.