Add an arg param. If there is no such param in .caffemodel fie, silently ignore it.

def add_optional_arg_param(self, param_name, layer_index, blob_index):
        """Add an arg param. If there is no such param in .caffemodel fie, silently ignore it."""
        blobs = self.layers[layer_index].blobs
        if blob_index < len(blobs):
            self.add_arg_param(param_name, layer_index, blob_index)