message: [XLA:GPU] Add error message to CHECK for preconditions to lower fusions with multiple reduce outputs.
diff: @@ -2443,8 +2443,11 @@ StatusOr<std::unique_ptr<Thunk>> IrEmitterUnnested::BuildInitializerThunk(
  case HloOpcode::kReduce:
  return inst->operand(1);
  case HloOpcode::kTuple:
  - CHECK(hlo->IsMultiOutputFusion() &&
  - inst->operand(index.back())->opcode() == HloOpcode::kReduce);
  + CHECK(hlo->IsMultiOutputFusion())
  + << ": " << hlo->ToString() << " is not a multi-output fusion.";
  + CHECK(inst->operand(index.back())->opcode() == HloOpcode::kReduce)
  + << ": Found '" << inst->operand(index.back())->opcode() << "' in "
  + << inst->ToString() << " but expected 'reduce'.";
  // For multi-output fusion look through the tuple.
  return inst->operand(index.back())->operand(1);
  default:
  