[GSoC] Train DeBERTa NER model for required phrase prediction

[Kaushik-Kumar-CEG]

continuation of #5077. uses the BIOES dataset produced there

Goal

Finetune DeBERTa-v3-large to predict required phrase spans in license rule text using BIOES labels

Tasks

• Subword tokenization with label alignment (-100 for continuation tokens)
• Training with class-weighted loss (handle O vs B/I/E/S imbalance)
• Include negative samples (rules without markers, all-O labels) for balanced training
• Evaluation: token F1, exact span match
• ONNX export for CPU inference