Resolve the FP4 attention kernel and backend conflicts with main's #1980 versions (re-adding install_fp4_vsa_attention), keep main's FP32 FSDP grouping, and keep both save_inference and save_tagged in the checkpointer.
Resolve the FP4 attention kernel and backend conflicts with main's #1980 versions (re-adding install_fp4_vsa_attention), keep main's FP32 FSDP grouping, and keep both save_inference and save_tagged in the checkpointer.