[feat] Add support for encoder-only transformers (e.g. BERT)

[OxxoCodes]

🚀 The feature, motivation and pitch

Liger Kernel is currently incompatible with encoder-only transformer architectures such as BERT, DistilBERT, RoBERTa, XLM-R, and DeBERTa.

Given the importance these models still have in research and industry use-cases, it would be great to see support added to further decrease memory requirements and increase training throughput.

Alternatives

No response

Additional context

No response