[ENH] Add warning for multi-target forecasting support in BaseModel

[PranavBhatP]

Reference Issues/PRs

fixes #1879.

What does this implement/fix? Explain your changes.

Additional warning for missing multi-target support.

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

[codecov]

Codecov Report

Attention: Patch coverage is 66.66667% with 1 line in your changes missing coverage. Please review.

Please upload report for BASE (main@57a6a1c). Learn more about missing BASE report.

Files with missing lines	Patch %	Lines
pytorch_forecasting/models/base/_base_model_v2.py	66.66%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1880   +/-   ##
=======================================
  Coverage        ?   87.78%           
=======================================
  Files           ?       68           
  Lines           ?     6573           
  Branches        ?        0           
=======================================
  Hits            ?     5770           
  Misses          ?      803           
  Partials        ?        0           

Flag	Coverage Δ
cpu	87.78% <66.66%> (?)
pytest	87.78% <66.66%> (?)

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[fkiraly]

See above