Prevent timestamps from decreasing in Whisper generation (#1063)

src/models/whisper.cc

@@ -436,6 +436,17 @@ namespace ctranslate2 {
             } else {
               check_timestamps_prob_for_batch.push_back(batch_id);
             }
+
+            // Timestamps shouldn't decrease: forbid timestamp tokens smaller than the last.
+            for (dim_t t = step - 1; t >= sample_begin; --t) {
+              const size_t token = sequences.at<int32_t>({batch_id, t});
+
+              if (token >= _timestamp_begin_id) {
+                for (size_t i = _timestamp_begin_id; i <= token; ++i)
+                  disable_tokens.add(batch_id, i);
+                break;
+              }
+            }
           }
         }