Feat/sample weight torch

[dennisbader]

Checklist before merging this PR:

• Mentioned all issues that this PR fixes or addresses.
• Summarized the updates of this PR under Summary.
• Added an entry under Unreleased in the Changelog.

Fixes #1175, fixes #2107.

(this is a continuation of #2404)

Summary

• adds support for training torch forecasting models with sample weights.
• fit() now has two new parameters sample_weight and val_sample_weight which allow to pass sample weights for training and validation

sample_weight
            Optionally, some sample weights to apply to the target `series` labels.
            They are applied per observation, per label (each step in `output_chunk_length`), and per component.
            If a string, then the weights are generated using built-in weighting functions. The available options are
            `"linear_decay"` or `"exponential_decay"`. The weights are only computed the longest series in `series`,
            and then applied globally to all `series` to have a common time weighting.
            If a `TimeSeries` or `Sequence[TimeSeries]`, then those weights are used. The number of series must
            match the number of target `series` and each series must contain at least all time steps from the
            corresponding target `series`. If the weight series only have a single component / column, then the weights
            are applied globally to all components in `series`. Otherwise, for component-specific weights, the number
            of components must match those of `series`.
val_sample_weight
            Same as for `sample_weight` but for the evaluation dataset.

[codecov]

Codecov Report

Attention: Patch coverage is 88.62559% with 24 lines in your changes missing coverage. Please review.

Project coverage is 93.73%. Comparing base (05f6ddf) to head (a1fd48c).
Report is 2 commits behind head on master.

❗ Current head a1fd48c differs from pull request most recent head c31dbc9

Please upload reports for the commit c31dbc9 to get more accurate results.

Files	Patch %	Lines
darts/utils/data/shifted_dataset.py	82.45%	10 Missing ⚠️
darts/utils/data/horizon_based_dataset.py	76.00%	6 Missing ⚠️
darts/utils/data/utils.py	84.61%	4 Missing ⚠️
...arts/models/forecasting/torch_forecasting_model.py	92.30%	2 Missing ⚠️
darts/utils/likelihood_models.py	91.30%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2410      +/-   ##
==========================================
- Coverage   93.77%   93.73%   -0.04%     
==========================================
  Files         138      138              
  Lines       14384    14632     +248     
==========================================
+ Hits        13488    13716     +228     
- Misses        896      916      +20     

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[madtoinou]

LGTM! 🚀