feat: training set SDK support (DE-8692)

[luke-e-schaefer]

Training Set SDK support (DE-8692)

Adds SDK support for training sets — a mutable, versioned, model-scoped collection of dataset_item ids spanning one or more source datasets. Similar to the Benchmark resource.

Ticket: DE-8692

What's added

• nucleus/training_set.py — TrainingSet dataclass: from_json, refresh, update, delete, items, add_items, remove_items, new_version, family, lineage fields.
• NucleusClient methods: create_training_set, get_training_set, list_training_sets, get_model_training_set, repin_training_set, update/delete_training_set, list/add/remove_training_set_items, create_training_set_version, list_training_set_family.
• Model entry points: Model.create_training_set(...), Model.training_set, Model.repin_training_set(...).

Greptile Summary

Adds model-scoped training-set SDK support, including mutable membership, version lineage, exports, and media downloads.

• Exposes training-set operations through NucleusClient, Model, and the top-level package.
• Adds paginated item export, JSONL serialization, and collision-safe per-dataset media naming.
• Adds transfer objects, constants, tests, changelog documentation, and the package version update.

Confidence Score: 5/5

The PR appears safe to merge because no blocking failure remains.

No blocking failure remains.

Important Files Changed

Filename	Overview
nucleus/init.py	Adds the client-level training-set API, payload construction, pagination, export hydration, mutation, versioning, and top-level exports.
nucleus/training_set.py	Implements the TrainingSet resource and collision-safe same-call export and download behavior; previously reported collision paths are addressed.
nucleus/model.py	Adds model-scoped convenience entry points for creating, reading, and repinning training sets.
nucleus/data_transfer_object/training_set.py	Adds the paginated training-set member response model.
tests/test_training_sets.py	Covers training-set lifecycle operations, pagination, exports, downloads, and the previously reported same-call filename collisions.

Flowchart

%%{init: {'theme': 'neutral'}}%%
flowchart LR
    App[SDK caller] --> Model[Model]
    App --> Client[NucleusClient]
    Model --> Client
    Client --> API[Training Set API]
    API --> Set[TrainingSet]
    Set --> Membership[Add or remove members]
    Set --> Version[Create version or repin]
    Set --> Export[Export records or download media]

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