Fixed Pallas Mosaic GPU test following recent changes

PiperOrigin-RevId: 679261897

jax/_src/pallas/mosaic_gpu/lowering.py

@@ -357,6 +357,10 @@ def body(launch_ctx: mgpu.LaunchContext, *buffers: ir.Value):
                   *aval.shape, dtype=mgpu_utils.dtype_to_ir_type(aval.dtype)
               )
           )
+        case gpu_core.AbstractMemoryRef() if isinstance(
+            aval.dtype, gpu_core.BarrierType
+        ):
+          pass
         case gpu_core.AbstractMemoryRef() if aval.memory_space == SMEM:
           scratch_buffers_template.append(next(smem_scratch_it))
           should_discharge.append(False)
@@ -397,7 +401,7 @@ def gmem_slice(
         step: ir.Value,
         block_mapping: pallas_core.BlockMapping,
     ) -> Sequence[mgpu.DynamicSlice]:
-      assert len(sequential_axes) == 1
+      assert len(sequential_axes) <= 1
       program_ids = [step if i is None else i for i in program_ids_template]
       idxs = _eval_index_map(module_ctx, launch_ctx, program_ids, block_mapping)
       return tuple(
@@ -428,7 +432,7 @@ def store(
         idx: int, step: ir.Value, slot: ir.Value, prev_base_offset: ir.Value
     ) -> ir.Value:
       if not out_in_smem[idx]:
-        return
+        return _as_index(-1)
 
       # We have to do some work to make sure that consecutive stores are not
       # going to be writing to the same location, or else we'll end up with