🚨🚨🚨 `[NLLB Tokenizer]` Fix the prefix tokens 🚨🚨🚨 (#22313)

* fix the prefix tokens

* update fast and test values

* add legacy behaviour
Co-authored-by: sgugger <sylvain.gugger@gmail.com>

* update disclaimer, linkissue PR and behaviral changes

* Apply suggestions from code review
Co-authored-by: Lysandre Debut <hi@lysand.re>

* styling

* make a quote

* quote this time

---------
Co-authored-by: sgugger <sylvain.gugger@gmail.com>
Co-authored-by: Lysandre Debut <hi@lysand.re>

docs/source/en/model_doc/nllb.mdx

@@ -12,8 +12,45 @@ specific language governing permissions and limitations under the License.
 
 # NLLB
 
-**DISCLAIMER:** If you see something strange, file a [Github Issue](https://github.com/huggingface/transformers/issues/new?assignees=&labels=bug&template=bug-report.yml) and assign
-@LysandreJik
+**DISCLAIMER:** The default behaviour for the tokenizer has recently been fixed (and thus changed)!
+
+The previous version adds `[self.eos_token_id, self.cur_lang_code]` at the end of the token sequence for both target and source tokenization. This is wrong as the NLLB paper mentions (page 48, 6.1.1. Model Architecture) :
+
+*Note that we prefix the source sequence with the source language, as opposed to the target
+language as previously done in several works (Arivazhagan et al., 2019; Johnson et al.,
+2017). This is primarily because we prioritize optimizing zero-shot performance of our
+model on any pair of 200 languages at a minor cost to supervised performance.*
+
+Previous behaviour:
+
+```python
+>>> from transformers import NllbTokenizer
+
+>>> tokenizer = NllbTokenizer.from_pretrained("facebook/nllb-200-distilled-600M")
+>>> tokenizer("How was your day?").input_ids
+[13374, 1398, 4260, 4039, 248130, 2, 256047]
+
+>>> # 2: '</s>'
+>>> # 256047 : 'eng_Latn'
+```
+New behaviour
+
+```python
+>>> from transformers import NllbTokenizer
+
+>>> tokenizer = NllbTokenizer.from_pretrained("facebook/nllb-200-distilled-600M")
+>>> tokenizer("How was your day?").input_ids
+[256047, 13374, 1398, 4260, 4039, 248130, 2]
+ ```
+
+Enabling the old behaviour can be done as follows:
+```python
+>>> from transformers import NllbTokenizer
+
+>>> tokenizer = NllbTokenizer.from_pretrained("facebook/nllb-200-distilled-600M", legacy_behaviour=True)
+```
+
+For more details, feel free to check the linked [PR](https://github.com/huggingface/transformers/pull/22313) and [Issue](https://github.com/huggingface/transformers/issues/19943).
 
 ## Overview of NLLB
 

src/transformers/models/nllb/tokenization_nllb.py

@@ -140,12 +140,14 @@ class NllbTokenizer(PreTrainedTokenizer):
         tgt_lang=None,
         sp_model_kwargs: Optional[Dict[str, Any]] = None,
         additional_special_tokens=None,
+        legacy_behaviour=False,
         **kwargs,
     ):
         # Mask token behave like a normal word, i.e. include the space before it
         mask_token = AddedToken(mask_token, lstrip=True, rstrip=False) if isinstance(mask_token, str) else mask_token
 
         self.sp_model_kwargs = {} if sp_model_kwargs is None else sp_model_kwargs
+        self.legacy_behaviour = legacy_behaviour
 
         super().__init__(
             bos_token=bos_token,
@@ -160,13 +162,13 @@ class NllbTokenizer(PreTrainedTokenizer):
             tgt_lang=tgt_lang,
             additional_special_tokens=additional_special_tokens,
             sp_model_kwargs=self.sp_model_kwargs,
+            legacy_behaviour=legacy_behaviour,
             **kwargs,
         )
 
         self.sp_model = spm.SentencePieceProcessor(**self.sp_model_kwargs)
         self.sp_model.Load(str(vocab_file))
         self.vocab_file = vocab_file
-
         # Original fairseq vocab and spm vocab must be "aligned":
         # Vocab    |    0    |    1    |   2    |    3    |  4   |  5   |  6   |   7  |   8  |  9
         # -------- | ------- | ------- | ------ | ------- | ---- | ---- | ---- | ---- | ---- | ----
@@ -388,13 +390,27 @@ class NllbTokenizer(PreTrainedTokenizer):
         return self.set_tgt_lang_special_tokens(self.tgt_lang)
 
     def set_src_lang_special_tokens(self, src_lang) -> None:
-        """Reset the special tokens to the source lang setting. No prefix and suffix=[eos, src_lang_code]."""
+        """Reset the special tokens to the source lang setting.
+        - In legacy mode: No prefix and suffix=[eos, src_lang_code].
+        - In default mode: Prefix=[src_lang_code], suffix = [eos]
+        """
         self.cur_lang_code = self.lang_code_to_id[src_lang]
+        if self.legacy_behaviour:
             self.prefix_tokens = []
             self.suffix_tokens = [self.eos_token_id, self.cur_lang_code]
+        else:
+            self.prefix_tokens = [self.cur_lang_code]
+            self.suffix_tokens = [self.eos_token_id]
 
     def set_tgt_lang_special_tokens(self, lang: str) -> None:
-        """Reset the special tokens to the target language setting. No prefix and suffix=[eos, tgt_lang_code]."""
+        """Reset the special tokens to the target lang setting.
+        - In legacy mode: No prefix and suffix=[eos, tgt_lang_code].
+        - In default mode: Prefix=[tgt_lang_code], suffix = [eos]
+        """
         self.cur_lang_code = self.lang_code_to_id[lang]
+        if self.legacy_behaviour:
             self.prefix_tokens = []
             self.suffix_tokens = [self.eos_token_id, self.cur_lang_code]
+        else:
+            self.prefix_tokens = [self.cur_lang_code]
+            self.suffix_tokens = [self.eos_token_id]

src/transformers/models/nllb/tokenization_nllb_fast.py

@@ -151,11 +151,12 @@ class NllbTokenizerFast(PreTrainedTokenizerFast):
         src_lang=None,
         tgt_lang=None,
         additional_special_tokens=None,
+        legacy_behaviour=False,
         **kwargs,
     ):
         # Mask token behave like a normal word, i.e. include the space before it
         mask_token = AddedToken(mask_token, lstrip=True, rstrip=False) if isinstance(mask_token, str) else mask_token
-
+        self.legacy_behaviour = legacy_behaviour
         super().__init__(
             vocab_file=vocab_file,
             tokenizer_file=tokenizer_file,
@@ -169,6 +170,7 @@ class NllbTokenizerFast(PreTrainedTokenizerFast):
             src_lang=src_lang,
             tgt_lang=tgt_lang,
             additional_special_tokens=additional_special_tokens,
+            legacy_behaviour=legacy_behaviour,
             **kwargs,
         )
 
@@ -287,10 +289,18 @@ class NllbTokenizerFast(PreTrainedTokenizerFast):
         return self.set_tgt_lang_special_tokens(self.tgt_lang)
 
     def set_src_lang_special_tokens(self, src_lang) -> None:
-        """Reset the special tokens to the source lang setting. No prefix and suffix=[eos, src_lang_code]."""
+        """Reset the special tokens to the source lang setting.
+        - In legacy mode: No prefix and suffix=[eos, src_lang_code].
+        - In default mode: Prefix=[src_lang_code], suffix = [eos]
+        """
         self.cur_lang_code = self.convert_tokens_to_ids(src_lang)
+
+        if self.legacy_behaviour:
             self.prefix_tokens = []
             self.suffix_tokens = [self.eos_token_id, self.cur_lang_code]
+        else:
+            self.prefix_tokens = [self.cur_lang_code]
+            self.suffix_tokens = [self.eos_token_id]
 
         prefix_tokens_str = self.convert_ids_to_tokens(self.prefix_tokens)
         suffix_tokens_str = self.convert_ids_to_tokens(self.suffix_tokens)
@@ -302,10 +312,17 @@ class NllbTokenizerFast(PreTrainedTokenizerFast):
         )
 
     def set_tgt_lang_special_tokens(self, lang: str) -> None:
-        """Reset the special tokens to the target language setting. No prefix and suffix=[eos, tgt_lang_code]."""
+        """Reset the special tokens to the target lang setting.
+        - In legacy mode: No prefix and suffix=[eos, tgt_lang_code].
+        - In default mode: Prefix=[tgt_lang_code], suffix = [eos]
+        """
         self.cur_lang_code = self.convert_tokens_to_ids(lang)
+        if self.legacy_behaviour:
             self.prefix_tokens = []
             self.suffix_tokens = [self.eos_token_id, self.cur_lang_code]
+        else:
+            self.prefix_tokens = [self.cur_lang_code]
+            self.suffix_tokens = [self.eos_token_id]
 
         prefix_tokens_str = self.convert_ids_to_tokens(self.prefix_tokens)
         suffix_tokens_str = self.convert_ids_to_tokens(self.suffix_tokens)

tests/models/nllb/test_tokenization_nllb.py

@@ -305,6 +305,7 @@ class NllbDistilledIntegrationTest(unittest.TestCase):
         " face decât să înrăutăţească violenţele şi mizeria pentru milioane de oameni.",
     ]
     expected_src_tokens = [
+        256047,
         16297,
         134408,
         8165,
@@ -319,7 +320,6 @@ class NllbDistilledIntegrationTest(unittest.TestCase):
         108,
         49486,
         2,
-        256047,
     ]
 
     @classmethod
@@ -355,8 +355,8 @@ class NllbDistilledIntegrationTest(unittest.TestCase):
         assert isinstance(src_text[0], str)
         desired_max_length = 10
         ids = self.tokenizer(src_text, max_length=desired_max_length, truncation=True).input_ids[0]
-        self.assertEqual(ids[-2], 2)
-        self.assertEqual(ids[-1], EN_CODE)
+        self.assertEqual(ids[-1], 2)
+        self.assertEqual(ids[0], EN_CODE)
         self.assertEqual(len(ids), desired_max_length)
 
     def test_mask_token(self):
@@ -389,10 +389,10 @@ class NllbDistilledIntegrationTest(unittest.TestCase):
         self.assertEqual((2, 15), batch.attention_mask.shape)
         result = batch.input_ids.tolist()[0]
         self.assertListEqual(self.expected_src_tokens, result)
-        self.assertEqual(2, batch.decoder_input_ids[0, -1])  # EOS
+        self.assertEqual(RO_CODE, batch.decoder_input_ids[0, 0])  # EOS
         # Test that special tokens are reset
-        self.assertEqual(self.tokenizer.prefix_tokens, [])
-        self.assertEqual(self.tokenizer.suffix_tokens, [self.tokenizer.eos_token_id, EN_CODE])
+        self.assertEqual(self.tokenizer.prefix_tokens, [EN_CODE])
+        self.assertEqual(self.tokenizer.suffix_tokens, [self.tokenizer.eos_token_id])
 
     def test_seq2seq_max_length(self):
         batch = self.tokenizer(self.src_text, padding=True, truncation=True, max_length=3, return_tensors="pt")
@@ -419,9 +419,27 @@ class NllbDistilledIntegrationTest(unittest.TestCase):
             nested_simplify(inputs),
             {
                 # A, test, EOS, en_XX
-                "input_ids": [[70, 7356, 2, 256047]],
+                "input_ids": [[256047, 70, 7356, 2]],
                 "attention_mask": [[1, 1, 1, 1]],
                 # ar_AR
                 "forced_bos_token_id": 256057,
             },
         )
+
+    @require_torch
+    def test_legacy_behaviour(self):
+        self.tokenizer.legacy_behaviour = True
+        inputs = self.tokenizer(
+            "UN Chief says there is no military solution in Syria", src_lang="eng_Latn", tgt_lang="fra_Latn"
+        )
+        self.assertEqual(
+            inputs.input_ids, [16297, 134408, 25653, 6370, 248, 254, 103929, 94995, 108, 49486, 2, 256047]
+        )
+
+        self.tokenizer.legacy_behaviour = False
+        inputs = self.tokenizer(
+            "UN Chief says there is no military solution in Syria", src_lang="eng_Latn", tgt_lang="fra_Latn"
+        )
+        self.assertEqual(
+            inputs.input_ids, [256047, 16297, 134408, 25653, 6370, 248, 254, 103929, 94995, 108, 49486, 2]
+        )