Better heuristic for token-classification pipeline. (#12611)

* Better heuristic for token-classification pipeline.

Relooking at the problem makes thing actually much simpler,
when we look at ids from a tokenizer, we have no way in **general**
to recover if some substring is part of a word or not.

However, within the pipeline, with offsets we still have access to the
original string, so we can simply look if previous character (if it
exists) of a token, is actually a space. This will obviously be wrong
for tokenizers that contain spaces within tokens, tokenizers where
offsets include spaces too (Don't think there are a lot).

This heuristic hopefully is fully bc and still can handle non-word based
tokenizers.

* Updating test with real values.

* We still need the older "correct" heuristic to prevent fusing
punctuation.

* Adding a real warning when important.

src/transformers/pipelines/token_classification.py

@@ -270,7 +270,19 @@ class TokenClassificationPipeline(Pipeline):
             if offset_mapping is not None:
                 start_ind, end_ind = offset_mapping[idx]
                 word_ref = sentence[start_ind:end_ind]
-                is_subword = len(word_ref) != len(word)
+                if getattr(self.tokenizer._tokenizer.model, "continuing_subword_prefix", None):
+                    # This is a BPE, word aware tokenizer, there is a correct way
+                    # to fuse tokens
+                    is_subword = len(word) != len(word_ref)
+                else:
+                    # This is a fallback heuristic. This will fail most likely on any kind of text + punctuation mixtures that will be considered "words". Non word aware models cannot do better than this unfortunately.
+                    if self.aggregation_strategy in {
+                        AggregationStrategy.FIRST,
+                        AggregationStrategy.AVERAGE,
+                        AggregationStrategy.MAX,
+                    }:
+                        warnings.warn(UserWarning, "Tokenizer does not support real words, using fallback heuristic")
+                    is_subword = sentence[start_ind - 1 : start_ind] != " " if start_ind > 0 else False
 
                 if int(input_ids[idx]) == self.tokenizer.unk_token_id:
                     word = word_ref

tests/test_pipelines_token_classification.py

@@ -191,6 +191,19 @@ class TokenClassificationPipelineTests(CustomInputPipelineCommonMixin, unittest.
             ],
         )
 
+    @require_torch
+    @slow
+    def test_aggregation_strategy_byte_level_tokenizer(self):
+        sentence = "Groenlinks praat over Schiphol."
+        ner = pipeline("ner", model="xlm-roberta-large-finetuned-conll02-dutch", aggregation_strategy="max")
+        self.assertEqual(
+            nested_simplify(ner(sentence)),
+            [
+                {"end": 10, "entity_group": "ORG", "score": 0.994, "start": 0, "word": "Groenlinks"},
+                {"entity_group": "LOC", "score": 1.0, "word": "Schiphol.", "start": 22, "end": 31},
+            ],
+        )
+
     @require_torch
     def test_aggregation_strategy(self):
         model_name = self.small_models[0]