Function gru_forward_sequence 

pub fn gru_forward_sequence(
    cell: &GruCell,
    input: &Tensor,
    h0: Option<&Tensor>,
) -> Result<(Tensor, Tensor), ModelError>

Runs a GRU cell over a sequence [batch, seq_len, input_size].

Returns all hidden states [batch, seq_len, hidden_size] and final hidden [batch, hidden_size].