code: def get_output_from_input(self):
          
          input_path = self.layer.currentLayer().source()
          output_path = (
              os.path.splitext(input_path)[0] + '_multi_buffer'
              + os.path.splitext(input_path)[1])
          self.output_form.setText(output_path)
docs: Populate output form with default output path based on input layer.