v2.26.0: resolve trainer attention in one helper

Features

• resolve trainer attention in one helper

  resolve_attn_implementation now picks the trainer attention backend in one place. Order: an explicit value, then ATTN_IMPLEMENTATION / AGILERL_ATTN_IMPLEMENTATION, then the family trainer default (Gemma sliding-window flex_attention, Nemotron-H flash_attention_2), then flash_attention_2 if flash_attn is installed otherwise sdpa.

  LLMBuilder and create_model_from_name_or_path both go through that helper. Nemotron-H's catalog trainer config sets attn_implementation to flash_attention_2.

Full Changelog: v2.25.0...v2.26.0