examples: add multi-label text classification (BCEWithLogitsLoss, met…

[aryaMehta26]

…rics, threshold tuning) + README usage

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Fixes # (issue)

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[aryaMehta26]

Adds a minimal multi-label text classification example (BCEWithLogitsLoss + threshold tuning) since multi-label is underrepresented in examples.

[Rocketknight1]

I'm not sure we want this right now, I'm sorry! Making multi-label classification accessible to newcomers is tricky, and probably needs a big README and code that's much more accessible than this, with fewer things like function wrappers and single-letter variables. And maybe no huge block that just tunes the probability threshold to boost the F1 score, lol

Though I appreciate you using a better code agent for this than most of the Copilot slop we get 😅