Function ctc_loss 

pub fn ctc_loss(
    graph: &mut Graph,
    log_probs: NodeId,
    targets: NodeId,
    input_lengths: NodeId,
    target_lengths: NodeId,
    blank: usize,
) -> Result<NodeId, ModelError>

CTC (Connectionist Temporal Classification) loss.

Simplified implementation for sequence-to-sequence tasks (OCR/ASR). log_probs: [T, batch, classes], targets: [batch, S], lengths as 1-D tensors.