[Codefuse开源轻训营] Support for Matryoshka Representation Learning (MRL) training strategy

[elvis-t9]

Add support for Matryoshka Representation Learning (MRL) - the "Russian Doll" training approach that enables flexible embedding dimensions at inference time. This technique allows a single model to produce high-quality embeddings at multiple dimensions (e.g., 64, 128, 256, 512, 1024), providing significant flexibility for different downstream applications and computational budgets.