[BUG] fixes correct passing of index in v2

[phoeenniixx]

Fixes #2367

Adds a new prediction_start_idx to the collate_fn that is returned when return_info has index.

[phoeenniixx]

Has to remove index from the cell in the tutorial as previously it was just printing y[1] (a random placeholder, see here). We still need to add some thing like x_to_index of v1, to make this index actually useful. But for now, it is similar to v1, matching group with decoder_idx (see x_to_index).

But this breaks what the cell printed before, as now preds["index"] is a 2d tensor and not 1d. So, i just removed it, it makes no sense there.

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[phoeenniixx]

The big diff in the tutorial is because I had to rerun the whole nb locally to update the outputs :)

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Codecov Report

❌ Patch coverage is 84.61538% with 2 lines in your changes missing coverage. Please review.
⚠️ Please upload report for BASE (main@033169a). Learn more about missing BASE report.

Files with missing lines	Patch %	Lines
pytorch_forecasting/callbacks/predict.py	81.81%	2 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2369   +/-   ##
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  Coverage        ?   87.45%           
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  Files           ?      175           
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  Branches        ?        0           
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  Hits            ?     8899           
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  Partials        ?        0           

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