Fix x.all_gather hint: use (1, block_n) for correct vectorization with partial-width tiles

iris/x/all_gather.py

@@ -64,13 +64,18 @@ def all_gather(
             # Scatter along N dimension: write to [:, ctx.rank * N_local : (ctx.rank+1) * N_local]
             dst_ptr, combined_mask = dst_view.offset_tile_ptr(tile, offset_n=ctx.rank * N_local, src_mask=None)
 
-        # Use iris.store to write to dest_rank's memory
+        # Use iris.store to write to dest_rank's memory.
+        # hint=(1, tile.block_n) asserts per-row contiguity only (BLOCK_N consecutive
+        # elements within each row).  Using (tile.block_m, tile.block_n) would
+        # assert cross-row contiguity which is false when BLOCK_N < N (stride_m > BLOCK_N),
+        # causing getOrderFromContiguity to choose dim-0 for vectorization and emitting
+        # scalar buffer_store_short writes to wrong addresses.
         iris.store(
             dst_ptr,
             tile.data,
             ctx.rank,  # from_rank (current rank)
             dest_rank,  # to_rank (destination rank)
             ctx.heap_bases,
             mask=combined_mask,
-            hint=(tile.block_m, tile.block_n),
+            hint=(1, tile.block_n),
         )