src/transformers/models/reformer/modeling_reformer.py/LocalSelfAttention/_retrieve_relevant_hidden_states
class LocalSelfAttention:    def _retrieve_relevant_hidden_states(previous_hidden_states, chunk_length, num_chunks_before):
        start_position = ((previous_hidden_states.shape[1] // chunk_length) - num_chunks_before) * chunk_length
        return previous_hidden_states[:, start_position:]