[MNT] Improve error message for invalid reduction argument in groupby_apply

[Gyanam1310]

Reference Issues/PRs

None.

What does this implement/fix? Explain your changes.

This PR improves the error message raised when an invalid reduction
argument is passed to groupby_apply.

Previously, the error message only stated that the reduction was unknown.
This update clarifies the expected valid options: {'mean', 'sum'}.

No functional behavior is changed.

What should a reviewer concentrate their feedback on?

• Clarity and wording of the updated error message.
• Whether the expected options are correctly specified.

Did you add any tests for the change?

No. This change only improves the error message wording and does not
modify functionality.

Any other comments?

This is a minor maintenance improvement to make debugging clearer
for users passing invalid arguments.

[codecov]

Codecov Report

❌ Patch coverage is 0% with 1 line in your changes missing coverage. Please review.
⚠️ Please upload report for BASE (main@6b27898). Learn more about missing BASE report.

Files with missing lines	Patch %	Lines
pytorch_forecasting/utils/_utils.py	0.00%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2104   +/-   ##
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  Coverage        ?   86.63%           
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  Files           ?      165           
  Lines           ?     9732           
  Branches        ?        0           
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  Hits            ?     8431           
  Misses          ?     1301           
  Partials        ?        0           

Flag	Coverage Δ
cpu	86.63% <0.00%> (?)
pytest	86.63% <0.00%> (?)

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[phoeenniixx]

Thanks!