[AMD] Fix Always mask padded topk_ids on HIP to prevent garbage MoE routing (DeepSeek-R1-MXFP4 accuracy regression) (#28378)

Co-authored-by: kangwangamd <kangwangamd@users.noreply.github.com>

python/sglang/srt/layers/moe/topk.py

@@ -1569,17 +1569,18 @@ def _post_process_topk_ids(
                 topk_ids, expert_location_dispatch_info, num_token_non_padded
             )
     elif _is_hip:
+        # On AMD HIP the aiter MoE kernels do not handle topk_ids=-1 safely
+        # (negative indices cause illegal memory access). Always fill the padded
+        # region with 0 so every kernel sees a valid in-range expert id.
+        # Routing weights for padded tokens are zeroed below so their
+        # contribution to the hidden state is still zero regardless of the id.
+        # Regression: skipping this mask when EPLB is disabled caused garbage
+        # MoE routing for models like DeepSeek-R1-MXFP4 (accuracy ~0.09 vs 0.94+).
+        _mask_topk_ids_padded_region(topk_ids, num_token_non_padded, fill_value=0)
         # The logical->physical remap is only meaningful when a real
         # expert-location mapping exists. With a trivial placement and EPLB off
-        # the map is identity, and indexing it here is both unnecessary and not
-        # well-defined for the dp-attention padded region of topk_ids; skip it.
-        # ``--ep-dispatch-algorithm fake`` is a routing benchmark artifact and
-        # does not by itself require remapping.
+        # the map is identity so the remap can be skipped safely.
         if _eplb_remap_enabled():
-            # Mask the padded region to a valid in-range id (0) before the
-            # gather so it never indexes the dispatch map out of bounds. -1 is
-            # not used here: the aiter MoE kernels do not handle topk_ids=-1.
-            _mask_topk_ids_padded_region(topk_ids, num_token_non_padded, fill_value=0)
             topk_ids = topk_ids_logical_to_physical(
                 topk_ids, expert_location_dispatch_info
             )