fix backtracking when losses stay at inf (fixes #2206) (#2209)

* fix backtracking when losses stay at inf

* always backtrack when there is an inf value

dlib/dnn/trainer.h

@@ -1113,10 +1113,10 @@ namespace dlib
             while (previous_loss_values_to_keep_until_disk_sync.size() > 2 * gradient_updates_since_last_sync)
                 previous_loss_values_to_keep_until_disk_sync.pop_front();
 
-            // Always retry if there are any nan values
+            // Always retry if there are any nan or inf values
             for (auto x : previous_loss_values_to_keep_until_disk_sync)
             {
-                if (std::isnan(x))
+                if (std::isnan(x) || std::isinf(x))
                     return true;
             }