Function bilstm_forward_sequence 

pub fn bilstm_forward_sequence(
    fwd_cell: &LstmCell,
    bwd_cell: &LstmCell,
    input: &Tensor,
) -> Result<Tensor, ModelError>

Bidirectional LSTM: runs forward and backward LSTMs, concatenates outputs.

Returns [batch, seq_len, 2 * hidden_size].