[Model] Avoid hardcoding pooling type

[DarkLight1337]

Purpose

Gracefully handle user-specified pooling types where possible, instead of silently ignoring them.

In the next PR, we will work on passing EmbeddingPoolerHead to head argument so that pooling params are applied correctly.

Test Plan

Test Result

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Note

Enables user-configurable sequence pooling types instead of hardcoded defaults and aligns pooler construction across models.

• BERT/BertWithRope: BertPooler now takes seq_pooling_type (via get_seq_pooling_method); pooling initialized from pooler_config.
• ModernBert: ModernBertPooler uses configured seq_pooling_type and validates against HF classifier_pooling; sequence classification uses this pooler.
• RoBERTa and Transformers mixins: remove explicit CLSPool usage; DispatchPooler.for_seq_cls/for_embedding handle token extraction; minor comment clarifications.
• GritLM: use custom GritLMPooler only for MEAN; otherwise defer to generic pooler_for_embed.
• SPLADE embedding: note that built-in sequence pooling is overridden by SPLADESparsePooler.

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Note

Enables configurable sequence pooling and aligns pooler construction across models.

• BERT/BertWithRope: BertPooler now accepts seq_pooling_type and uses get_seq_pooling_method; BertPoolingModel/BertWithRope initialize pooler from pooler_config.
• ModernBert: ModernBertPooler uses provided seq_pooling_type; ModernBertForSequenceClassification validates HF classifier_pooling vs. configured type and wires pooling into DispatchPooler.for_seq_cls.
• RoBERTa + transformers mixins: Remove explicit CLSPool; rely on DispatchPooler.for_seq_cls/for_embedding for token extraction; clarify comments.
• GritLM: Use custom GritLMMeanPool only when seq_pooling_type == "MEAN"; otherwise defer to get_seq_pooling_method.
• SPLADE embedding: Note built-in sequence pooling is overridden by SPLADESparsePooler.

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Note

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Note

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Note

Makes sequence pooling user-configurable and standardizes pooler construction.

• BERT/BertWithRope: BertPooler now takes PoolerConfig and uses get_seq_pooling_method; pooling instantiated from pooler_config in BertPoolingModel and BertWithRope (when present)
• ModernBert: new ModernBertPooler(config, pooler_config) (uses HF classifier_pooling); ModernBertForSequenceClassification wires this through DispatchPooler.for_seq_cls
• RoBERTa + transformers mixins: remove explicit CLSPool; rely on DispatchPooler.for_seq_cls/for_embedding for token extraction; update default attribute to default_seq_pooling_type
• GritLM: GritLMPooler uses custom GritLMMeanPool only for MEAN, otherwise defers to get_seq_pooling_method; updates DispatchPooler wiring
• SPLADE embedding: note that built-in sequence pooling is overridden by SPLADESparsePooler

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[gemini-code-assist]

Code Review

This pull request effectively refactors the model implementations to avoid hardcoding the pooling type, instead sourcing it from the PoolerConfig. This is a great improvement for flexibility and configurability. The changes are consistent across various models like Bert, Roberta, and ModernBert. I've found one critical issue in bert_with_rope.py where an attribute might not be initialized, potentially leading to a runtime error. My detailed comment includes a fix for this.