fix routing to tiktoken encoder (#866)

Author: hwchase17

langchain/llms/openai.py

@@ -277,8 +277,13 @@ def get_num_tokens(self, text: str) -> int:
                 "This is needed in order to calculate get_num_tokens. "
                 "Please it install it with `pip install tiktoken`."
             )
+        encoder = "gpt2"
+        if self.model_name in ("text-davinci-003", "text-davinci-002"):
+            encoder = "p50k_base"
+        if self.model_name.startswith("code"):
+            encoder = "p50k_base"
         # create a GPT-3 encoder instance
-        enc = tiktoken.get_encoding("gpt2")
+        enc = tiktoken.get_encoding(encoder)
 
         # encode the text using the GPT-3 encoder
         tokenized_text = enc.encode(text)