message: Give more detail in error message for unsupported ops in XlaCompiler.
  Explains option for enabling soft_device_placement if an Unsupported op
  is encountered by XlaCompiler.
diff: @@ -1143,7 +1143,11 @@ Status ValidateGraph(const Graph* graph,
  return errors::InvalidArgument(absl::StrCat(
  "Detected unsupported operations when trying to compile graph ", name,
  " on ", device_type.type_string(), ": ", node->def().op(), " (",
  - s.error_message(), ")", FormatNodeForError(*node)));
  + s.error_message(), ")", FormatNodeForError(*node),
  + "One approach is to outside compile the unsupported ops to run on "
  + "CPUs by enabling soft placement "
  + "`tf.config.set_soft_device_placement(True)`."
  + " This has a potential performance penalty."));
  }
  return Status::OK();
  };
  