[Bugfix][LoRA] Use an explicit capability flag for tower connector LoRA

[linitra24]

get_num_mm_encoder_tokens is defined on SupportsMultiModal, so every model that inherits this interface has the method even if it does not implement it. Therefore, the previous hasattr check always returned True for multimodal models and could incorrectly enable tower connector LoRA.

Replace the hasattr(model, "get_num_mm_encoder_tokens") check with an explicit supports_tower_connector_lora capability flag.

Purpose

Test Plan

Test Result

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