add tot_score inf mask to make training stable (#2019)

I find there are some inf in tot_score, it makes model cannot converge, add inf mask can make training more stable.

Author: qweasdzxcvde

icefall/mmi.py

@@ -124,6 +124,7 @@ def _compute_mmi_loss_exact_non_optimized(
     den_tot_scores = den_lats.get_tot_scores(log_semiring=True, use_double_scores=True)
 
     tot_scores = num_tot_scores - den_scale * den_tot_scores
+    tot_scores = tot_scores.masked_fill(torch.isinf(tot_scores), 0.0)
 
     loss = -1 * tot_scores.sum()
     return loss