Add an SPM detokenizer that doesn't trim initial space (#681)

llms/mlx_lm/tokenizer_utils.py

@@ -1,4 +1,5 @@
 import json
+from functools import partial
 
 from transformers import AutoTokenizer
 
@@ -114,7 +115,9 @@ class SPMStreamingDetokenizer(StreamingDetokenizer):
     underscore which results in linear complexity.
     """
 
-    def __init__(self, tokenizer):
+    def __init__(self, tokenizer, trim_space=True):
+        self.trim_space = trim_space
+
         # Extract the tokens in a list from id to text
         self.tokenmap = [None] * len(tokenizer.vocab)
         for value, tokenid in tokenizer.vocab.items():
@@ -136,7 +139,7 @@ class SPMStreamingDetokenizer(StreamingDetokenizer):
     def add_token(self, token):
         v = self.tokenmap[token]
         if v[0] == "\u2581":
-            if self.text:
+            if self.text or not self.trim_space:
                 self.text += self._unflushed.replace("\u2581", " ")
             else:
                 self.text = _remove_space(self._unflushed.replace("\u2581", " "))
@@ -145,7 +148,7 @@ class SPMStreamingDetokenizer(StreamingDetokenizer):
             self._unflushed += v
 
     def finalize(self):
-        if self.text:
+        if self.text or not self.trim_space:
             self.text += self._unflushed.replace("\u2581", " ")
         else:
             self.text = _remove_space(self._unflushed.replace("\u2581", " "))
@@ -276,6 +279,18 @@ def _is_spm_decoder(decoder):
     return _match(_target_description, decoder)
 
 
+def _is_spm_decoder_no_space(decoder):
+    _target_description = {
+        "type": "Sequence",
+        "decoders": [
+            {"type": "Replace", "pattern": {"String": "▁"}, "content": " "},
+            {"type": "ByteFallback"},
+            {"type": "Fuse"},
+        ],
+    }
+    return _match(_target_description, decoder)
+
+
 def _is_bpe_decoder(decoder):
     _target_description = {
         "type": "ByteLevel",
@@ -302,6 +317,8 @@ def load_tokenizer(model_path, tokenizer_config_extra={}):
         if "decoder" in tokenizer_content:
             if _is_spm_decoder(tokenizer_content["decoder"]):
                 detokenizer_class = SPMStreamingDetokenizer
+            elif _is_spm_decoder_no_space(tokenizer_content["decoder"]):
+                detokenizer_class = partial(SPMStreamingDetokenizer, trim_space=False)
             elif _is_bpe_decoder(tokenizer_content["decoder"]):
                 detokenizer_class = BPEStreamingDetokenizer