message: Add def line before raising error in gen code
diff: @@ -666,13 +666,14 @@ bool GenEagerPythonOp::AddEagerFastPathAndGraphCode(
  bool GenEagerPythonOp::AddEagerFallbackCode(
  const string& parameters, const std::vector<string>& output_sizes,
  const string& num_outputs_expr, const string& eager_not_allowed_error) {
  + AddDefLine(strings::StrCat(function_name_, kEagerFallbackSuffix),
  + strings::StrCat(parameters, ", ctx=None"));
  +
  if (!eager_not_allowed_error.empty()) {
  strings::StrAppend(&result_, " ", eager_not_allowed_error);
  return true;
  }
  - AddDefLine(strings::StrCat(function_name_, kEagerFallbackSuffix),
  - strings::StrCat(parameters, ", ctx=None"));
  strings::StrAppend(
  &result_, " r\"\"\"This is the slowpath function for Eager mode.\n");
  strings::StrAppend(&result_, " This is for function ", function_name_,
  