175. Next Sentence Prediction Loss

Implement BERT's next sentence prediction (NSP) loss — a binary cross-entropy over the [CLS] logits.

Signature: def nsp_loss(cls_logits: np.ndarray, labels: np.ndarray) -> float

• cls_logits: shape (batch, 2)
• labels: shape (batch,) with values in {0, 1}

Return the mean cross-entropy. Use a numerically stable softmax.