Fix TiktokenTokenizer missing num_special_tokens_to_add (#30643)

Co-authored-by: Sushil Dubey <sushildubey171@users.noreply.github.com>
Co-authored-by: Mohammad Miadh Angkad <176301910+mmangkad@users.noreply.github.com>

python/sglang/srt/tokenizer/tiktoken_tokenizer.py

@@ -113,6 +113,10 @@ class TiktokenTokenizer:
     def decode(self, x, *args, **kwargs):
         return self.tokenizer.decode(x)
 
+    def num_special_tokens_to_add(self, *args, **kwargs) -> int:
+        # tiktoken's encode adds no special tokens (add_special_tokens is ignored).
+        return 0
+
     def batch_decode(
         self, batch, skip_special_tokens=True, spaces_between_special_tokens=False
     ):