Simplify benchmark to 3 modes: BF16 graph, NVFP4 pipeline graph, BF16 eager

Remove modes 3/4 (eager NVFP4, GEMM-only graph) that caused s_state
conflicts. Mode 5 (full pipeline graph) is now mode 2 — the only NVFP4
mode — and inits the GEMM with its own pointers without any prior clobber.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: TimDettmers