Add warning for stop string edge case (huggingface#33169)

* Add warning for edge case

* make fixup

src/transformers/generation/stopping_criteria.py

@@ -348,7 +348,14 @@ def _stop_string_create_embedding_vec(token_list, token_indices, stop_strings) -
         # we need a fallback to handle this case
         max_valid_positions = max(all_valid_positions) if all_valid_positions else 1
         # There should always be at least one valid end_len, however, so no fallback needed here
-        max_valid_end_lens = max(len(val) for positions in token_end_overlaps.values() for val in positions.values())
+        valid_end_lens = [len(val) for positions in token_end_overlaps.values() for val in positions.values()]
+        if not valid_end_lens:
+            raise ValueError(
+                "Stop string preprocessing was unable to identify tokens matching one or more of the "
+                "supplied stop string(s). This is most often caused by the stop "
+                "strings containing unusual characters that are not in the tokenizer vocabulary."
+            )
+        max_valid_end_lens = max(valid_end_lens)
         vec_size = len(stop_strings) * (max_valid_positions + max_valid_end_lens) + 1
         gather_vec = np.full((len(token_list), vec_size), dtype=np.int32, fill_value=-1)