docs: document ContrastiveTrainer single-dataset fixes in changelog

Author: tonywu71

CHANGELOG.md

@@ -19,6 +19,8 @@ and this project adheres to [Semantic Versioning](http://semver.org/).
 ### Fixed
 
 - Fix ModernVBERT wrappers to rely on the upstream Hugging Face implementation and keep checkpoint key conversion mapping working with current Transformers v5 loading.
+- Fix `ContrastiveTrainer._get_train_sampler` to accept the dataset argument that Transformers v5 passes positionally (single-dataset training crashed with a `TypeError` at dataloader build).
+- Fix `ContrastiveTrainer` to prime `query_prefix`/`pos_prefix`/`neg_prefix` from the collator in `__init__` (single-dataset training crashed with an `AttributeError` in `compute_loss`, as only the multi-dataset path set them).
 
 ## [0.3.14] - 2026-02-24