Fixes #45712

What does this PR do?

Removes six dead dummy classes from dummy_pt_objects.py:
BeamScorer, ConstrainedBeamSearchScorer, Constraint, ConstraintListState, DisjunctiveConstraint, PhrasalConstraint.

The real implementations were removed in v5 (#41223) but the dummies stayed. None of the six is referenced anywhere under src/, tests/, docs/, or in MIGRATION_GUIDE_V5.md. Without torch they leak into dir(transformers) and utils/check_repo.py fails the public-init-vs-docs invariant.

After removal, from transformers import BeamScorer raises ImportError in a no-torch env (matching the with-torch behavior) instead of returning the dummy and producing a misleading requires backend torch error on use.

DEPRECATED_OBJECTS in check_repo.py would be the alternative, but that list is for objects that still exist as aliases or shims (e.g. PretrainedConfig → PreTrainedConfig). These six don't.

Tests run (no-torch venv):

$ python utils/check_repo.py
# was: "Exception: ... in the public init, but not in the docs"
# now: docs step passes (later steps fail only on torch import, unrelated to this change)

$ python utils/check_dummies.py     # exit 0
$ python utils/check_inits.py       # exit 0
$ python utils/checkers.py copies,modular_conversion   # all pass

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• I confirm that this is not a pure code agent PR.

AI assistance (Claude) was used during investigation and patch preparation. All changes were reviewed and tested by me.

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• This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
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• Was this discussed/approved via a Github issue or the forum? Please add a link to it if that's the case. (Six leftover dummy classes in dummy_pt_objects.py fail check_repo.py and leak into dir(transformers) without torch Six leftover dummy classes in dummy_pt_objects.py fail check_repo.py and leak into dir(transformers) without torch #45712, approved by @Rocketknight1)
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