fix: revert invalid patches, add honest tuning surface assessment

REVERTED (invalid):
- paged_attn.py: restored use_v1=True hardcode. V2 is NotImplementedError
  on BI-V100, removing the guard would cause runtime crash.
- fused_moe.py: BLOCK_SIZE_N/K changes reverted. ixformer only reads
  BLOCK_SIZE_M from config dict, ignores N/K/GROUP_SIZE_M entirely
  (confirmed: _custom_ops.py:774 only passes config['BLOCK_SIZE_M']).
- _custom_ops.py: SMEM change reverted pending hardware confirmation.
- triton_flash_attention.py: autotune configs reverted (will re-add properly).
- prefix_prefill.py: comment enhancement reverted (was harmless but noisy).

ADDED:
- TUNING_SURFACE_TRUTH.md: honest assessment of what's actually tunable
  on BI-V100 with ixformer. Documents that bench_bi100.py benchmark
  functions are invalid (point params not injected into kernels).

Actual tuning surface is 5 parameters, not dozens:
  1. BLOCK_SIZE_M (fused_moe, passes to ixformer)
  2. use_v1 threshold (hardcoded True, V2 unimplemented)
  3. BLOCK/NUM_WARPS (prefix_prefill Triton JIT)
  4. SMEM declaration (affects Triton compiler)
  5. autotune config set (triton_flash_attention)

vllm/attention/ops/paged_attn.py

@@ -123,18 +123,9 @@ class PagedAttention:
         # to parallelize.
         # TODO(woosuk): Tune this heuristic.
         # For context len > 8192, use V2 kernel to avoid shared memory shortage.
-        # muh: BI-V100 (SM=16) V1/V2 heuristic
-        # V1: one CTA per (seq, head) — great for short seq, few SMs
-        # V2: partitioned — needed for long seq (>8192) to avoid SMEM overflow
-        # SM=16 means V1 has less parallelism to exploit, but V2's reduce
-        # overhead is proportionally higher. Keep V1 for longer than default.
-        # Original threshold: 8192. BI-V100: raise to 16384 (16K).
-        # If paged_attention_v2 is NotImplementedError on BI-V100, always V1.
-        try:
-            use_v1 = (max_seq_len <= 16384
-                      and (max_num_partitions == 1 or num_seqs * num_heads > 256))
-        except Exception:
-            use_v1 = True
+        use_v1 = (max_seq_len <= 8192
+                  and (max_num_partitions == 1 or num_seqs * num_heads > 512))
+        use_v1 = True
         if use_v1:
             # Run PagedAttention V1.
             ops.paged_attention_v1(