message: Added error message for unsupported uninitialized resource variables
  Previously, we ran into a generic error message when encountering unsupported
  uninitialized resource variables so it wasn't clear what the actual problem was.
diff: @@ -529,6 +529,12 @@ static StatusOr<std::vector<int>> RewriteWithArgs(
  mlir::BlockArgument mlir_arg = main_fn.getArgument(idx);
  if (xla_arg.kind == XlaArgument::kResource) {
  mlir::Type element_type;
  + if (xla_arg.type == DT_INVALID) {
  + return errors::Unimplemented(absl::StrCat(
  + "Argument ", idx,
  + " is an uninitialized resource variable which is currently"
  + " unsupported in the MLIR-based TPU bridge"));
  + }
  TF_RETURN_IF_ERROR(ConvertDataType(xla_arg.type, builder, &element_type));
  TF_ASSIGN_OR_RETURN(TensorShape arg_shape,
  GetTensorShapeFromXlaArgument(xla_arg));
  