fix models.llamacpp vocabulary normalization function

outlines/models/llamacpp.py

@@ -34,8 +34,10 @@ def __init__(self, model: "Llama"):
         self.tokenizer = model.tokenizer()
 
         # TODO: Remove when https://github.com/ggerganov/llama.cpp/pull/5613 is resolved
+        self._hf_tokenizer = None
         try:
             self.vocabulary = model.tokenizer_.hf_tokenizer.get_vocab()
+            self._hf_tokenizer = model.tokenizer_.hf_tokenizer
         except AttributeError:
             # ###
             for t in range(model.n_vocab()):
@@ -71,7 +73,15 @@ def encode(
         return token_ids, attention_mask
 
     def convert_token_to_string(self, token: str) -> str:
-        return token
+        if self._hf_tokenizer is not None:
+            from transformers.file_utils import SPIECE_UNDERLINE
+
+            token_str = self._hf_tokenizer.convert_tokens_to_string([token])
+            if token.startswith(SPIECE_UNDERLINE) or token == "<0x20>":
+                token_str = " " + token_str
+            return token_str
+        else:
+            return token
 
     def __eq__(self, other):
         if not isinstance(other, LlamaCppTokenizer):

tests/generate/test_integration_llamacpp.py

@@ -334,3 +334,25 @@ def test_RegexGuide_caching(model, temp_cache_dir):
     assert re.fullmatch(regex, structured)
     assert re.fullmatch(regex, structured_2)
     assert structured != structured_2
+
+
+def test_tokenizer_vocabulary_decode_sanity():
+    """Assert the decoded newline token (198) is the same as the normalized vocab token"""
+    import llama_cpp
+
+    model = models.llamacpp(
+        "bartowski/Meta-Llama-3-8B-Instruct-GGUF",
+        "Meta-Llama-3-8B-Instruct-IQ1_M.gguf",
+        tokenizer=llama_cpp.llama_tokenizer.LlamaHFTokenizer.from_pretrained(
+            "NousResearch/Hermes-2-Pro-Llama-3-8B"
+        ),
+    )
+    tokenizer = generate.regex(model, "a").logits_processor.tokenizer
+
+    decoded_nl_token = tokenizer.decode([198])[0]
+    vocab_nl_token = tokenizer.convert_token_to_string(
+        [token for token, token_id in tokenizer.vocabulary.items() if token_id == 198][
+            0
+        ]
+    )
+    assert decoded_nl_token == vocab_nl_token