//  [python]
def get_nll(
        self, seq_logits, doc_scores, target, reduce_loss=False, epsilon=0.0, exclude_bos_score=False, n_docs=None
    ):
        # shift tokens left
        target = tf.concat(
            [target[:, 1:], tf.fill([target.shape[0], 1], t.cast(self.config.generator.pad_token_id, target.dtype))],
            axis=1,
        )

        # bos_token_id is None for T5
        bos_token_id = self.config.bos_token_id or self.config.generator.bos_token_id
        n_docs = n_docs if n_docs is not None else self.config.n_docs
        equal_bos_token_id_all = tf.reduce_all(tf.equal(target[:, 0], bos_token_id))
        use_bos = bos_token_id is not None and equal_bos_token_id_all

        def _mask_pads(ll, smooth_obj):
            pad_mask = tf.equal(target, t.cast(self.config.generator.pad_token_id, target.dtype))
            if tf.reduce_any(pad_mask):
                ll = tf.where(pad_mask, 0.0, ll)
                smooth_obj = tf.where(pad_mask, 0.0, smooth_obj)
            return tf.squeeze(ll, axis=-1), tf.squeeze(smooth_obj, axis=-1)

        # seq_logits.shape = (batch*n_docs, tgt_len , vocabs)
        seq_logprobs = tf.nn.log_softmax(seq_logits, axis=-1)
        seq_logprobs = tf.reshape(
            seq_logprobs, (seq_logits.shape[0] // n_docs, n_docs, -1, seq_logits.shape[-1])
        )  # (batch_size, n_docs, tgt_len, vocabs)
        doc_logprobs = tf.nn.log_softmax(doc_scores, axis=1)
        doc_logprobs = tf.expand_dims(doc_logprobs, axis=-1)
        doc_logprobs = tf.expand_dims(doc_logprobs, axis=-1)  # done twice to get 4-D

        # RAG-sequence marginalization
        first_token_scores = seq_logprobs[:, :, :1, :]
        second_token_scores = seq_logprobs[:, :, 1:2, :]
        remainder = seq_logprobs[:, :, 2:, :]
        rag_logprobs = tf.concat([first_token_scores, second_token_scores + doc_logprobs, remainder], axis=2)

        # calculate loss
        target = tf.expand_dims(target, axis=1)  # n_docs dimension
        target = tf.expand_dims(target, axis=-1)  # logits dimension
        target = tf.repeat(target, n_docs, axis=1)
        assert len(target.shape) == len(rag_logprobs.shape)

        # last-axis gathering only - use 2D-reshape-trick for Torch's style nD gathering
        def torch_gather(param, id_tensor):
            # 2d-gather torch equivalent: https://stackoverflow.com/questions/52129909/tensorflow-equivalent-of-torch-gather
            def gather2d(target, id_tensor):
                idx = tf.stack([tf.range(tf.shape(id_tensor)[0], dtype=id_tensor.dtype), id_tensor[:, 0]], axis=-1)
                result = tf.gather_nd(target, idx)
                return tf.expand_dims(result, axis=-1)

            target = tf.reshape(param, (-1, param.shape[-1]))  # reshape 2D
            target_shape = id_tensor.shape

            id_tensor = tf.reshape(id_tensor, (-1, 1))  # also 2D-index
            result = gather2d(target, id_tensor)
            return tf.reshape(result, target_shape)

        ll = torch_gather(rag_logprobs, id_tensor=target)
        smooth_obj = tf.reduce_sum(rag_logprobs, axis=-1, keepdims=True)  # total sum of all (normalised) logits

        ll, smooth_obj = _mask_pads(ll, smooth_obj)

        # sum over tokens, exclude bos while scoring
        if exclude_bos_score and use_bos:
            ll = tf.reduce_sum(ll[:, :, 1:], axis=2)
        else:
            ll = tf.reduce_sum(ll, axis=2)

        smooth_obj = tf.reduce_sum(smooth_obj, axis=2)
        ll = tf.math.reduce_logsumexp(ll, axis=1)  # logsumexp over docs
        smooth_obj = tf.math.reduce_logsumexp(smooth_obj, axis=1)

        nll_loss = -ll
        smooth_loss = -smooth_obj

        if reduce_loss:
            nll_loss = tf.reduce_sum(nll_loss)
            smooth_loss = tf.reduce_sum(smooth_loss)

        eps_i = epsilon / rag_logprobs.shape[-1]
        loss = (1.0 - epsilon) * nll_loss + eps_i * smooth_loss
        return loss