[ENH] refactor _loss_mapping to tags of metrics, entirely

[Faakhir30]

Reference Issues/PRs

Closes #1957

What does this implement/fix? Explain your changes.

• refactor _loss_mapping.py to tags
• get compatible losses by filtering all_objects of type metric and specific metric_type based on pred_type and y_type
• move test kwargs for losses to metric pkg level
• introduced method get_test_train_params for this

PR checklist

• The PR title starts with either [ENH], [MNT], [DOC], or [BUG]. [BUG] - bugfix, [MNT] - CI, test framework, [ENH] - adding or improving code, [DOC] - writing or improving documentation or docstrings.
• Added/modified tests
• Used pre-commit hooks when committing to ensure that code is compliant with hooks. Install hooks with pre-commit install.
  To run hooks independent of commit, execute pre-commit run --all-files

[Faakhir30]

only updated Deeper model and BetaDistributionLoss pkg for now.
TODO: Will update remaining metric pkgs and model pkgs, after approval of design by maintainers

cc @phoeenniixx @fkiraly

Edit: Done, ready for revview

[codecov]

Codecov Report

❌ Patch coverage is 94.00000% with 3 lines in your changes missing coverage. Please review.
⚠️ Please upload report for BASE (main@4d8d97c). Learn more about missing BASE report.

Files with missing lines	Patch %	Lines
...ributions_pkg/_mqf2/_mqf2_distribution_loss_pkg.py	50.00%	2 Missing ⚠️
pytorch_forecasting/tests/test_all_estimators.py	96.00%	1 Missing ⚠️

Additional details and impacted files

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  Files           ?      166           
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Flag	Coverage Δ
cpu	87.53% <94.00%> (?)
pytest	87.53% <94.00%> (?)

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[phoeenniixx]

should we add a way to pass some other "params" to losses here?
I think that would make it overcomplicated no?
What do you think @Faakhir30 @fkiraly ?

[Faakhir30]

I think a solution will be to have some method that has dict of losses: loss_kwargs at model pkg level and testing framework override for that particular model+loss combination.

Currently I'm unable to see any such case, we can add it later if any such case arrive.

[phoeenniixx]

Nice!
I think it is ready! Just a small doubt and a suggestion (see above)
Please add docstrings to all public methods atleast - to prevent any confusions on what the methods do!