Fix implicit expansion when computing head logit mask

The implicit expansion emitted a Torch warning in each call.

syntaxdot-cli/src/subcommands/distill.rs

@@ -191,6 +191,8 @@ impl DistillApp {
         student_logits: &BiaffineScoreLogits,
         token_mask: &TokenMask,
     ) -> Result<Tensor> {
+        let (_, seq_len) = token_mask.size2()?;
+
         // Compute teacher probabilities.
         let teacher_head_probs = teacher_logits
             .head_score_logits
@@ -199,6 +201,7 @@ impl DistillApp {
         let probs_mask = token_mask
             .with_root()?
             .unsqueeze(1)
+            .f_expand([-1, seq_len, -1], true)?
             .logical_and(&token_mask.unsqueeze(-1));
         let teacher_head_probs = teacher_head_probs.masked_select(&probs_mask);