MAINT: Deprecate dead split kwarg on 8 single-split HF dataset loaders

[romanlutz]

Description

Eight _*Dataset loaders under pyrit/datasets/seed_datasets/remote/ expose a split: str constructor kwarg that forwards to _fetch_from_huggingface, even though the upstream HuggingFace dataset publishes only one split. The kwarg has exactly one valid value, never does anything useful, and is a misleading API surface that suggests users can pick a split when they can't.

Because the kwarg is part of the public constructor signature, this PR deprecates rather than deletes it (target removal: v0.16.0, matching the existing cohort in seed_dataset_provider.py and prepended_conversation_config.py):

• split: str = "<value>" → split: str | None = None
• DeprecationWarning(stacklevel=2) when a non-None value is passed
• self.split dropped; the single literal split name is hardcoded at the _fetch_from_huggingface(..., split="<value>") call site
• constructor docstring marks split as deprecated

Loaders affected (with the hardcoded literal):

Class	HF dataset	Literal
_CBTBenchDataset	Psychotherapy-LLM/CBT-Bench (38 configs, each single-split)	"train"
_DarkBenchDataset	apart/darkbench	"train"
_ForbiddenQuestionsDataset	TrustAIRLab/forbidden_question_set	config="default", split="train"
_HarmfulQADataset	declare-lab/HarmfulQA	"train"
_HiXSTestDataset	walledai/HiXSTest (gated)	"train"
_ORBench{80K,Hard,Toxic}Dataset	bench-llm/OR-Bench (one base-class edit)	"train"
_SGXSTestDataset	walledai/SGXSTest (gated)	"train"
_SimpleSafetyTestsDataset	Bertievidgen/SimpleSafetyTests	"test" (note: not "train")

_ForbiddenQuestionsDataset is a special case: the kwarg was actually misforwarded to HuggingFace as config= rather than split=, but TrustAIRLab/forbidden_question_set only has one config ("default") and one split ("train"), so the bug was harmless. The deprecation message explains both issues.

Out of scope: multi-split loaders where the split kwarg has more than one valid value — _BeaverTailsDataset, _SaladBenchDataset, _ToxicChatDataset, _JBBBehaviorsDataset. Whether those should be refactored into sibling subclasses per the "each distinct upstream artifact is its own registered subclass" rule is a separate design discussion.

Tests and Documentation

• Each affected test file gets a test_split_kwarg_emits_deprecation_warning case asserting the warning fires; existing tests that previously forwarded a non-default split= are updated to drop the kwarg (the hardcoded literal is still asserted on the _fetch_from_huggingface call kwargs).
• Unit tests: all 64 tests in the 8 affected test files pass, including the 9 new deprecation tests.
• End-to-end tests: tests/end_to_end/test_all_datasets.py was run scoped to the 10 affected providers (8 loader entries + 2 extra OR-Bench siblings) and all 10 successfully fetched real data from HuggingFace, including the 2 gated datasets (HiXSTest, SGXSTest) and the large OR-Bench 80K.
• Pre-commit (ruff format, ruff check, ty type check) passes on every modified file.
• Grep across pyrit/, tests/, and doc/ confirmed no other call sites pass split= to these loaders — only the new deprecation tests do.
• No documentation changes needed: the affected loaders are private classes (leading underscore), their constructor kwarg is not referenced from the rendered notebooks or other docs, and the deprecation message is self-documenting.