examples: add multi-label text classification (BCEWithLogitsLoss, met…
Author: aryaMehta26
Pull request: yes

…rics, threshold tuning) + README usage

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Comments:
Adds a minimal multi-label text classification example (BCEWithLogitsLoss + threshold tuning) since multi-label is underrepresented in examples.

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 :sweat_smile: