Allow use of gpt-2 and gpt-3.5 in encoding_for_model (#185)

Author: xhluca

tiktoken/model.py

@@ -20,6 +20,7 @@
     # chat
     "gpt-4": "cl100k_base",
     "gpt-3.5-turbo": "cl100k_base",
+    "gpt-3.5": "cl100k_base",  # Common shorthand
     "gpt-35-turbo": "cl100k_base",  # Azure deployment name
     # base
     "davinci-002": "cl100k_base",
@@ -63,6 +64,7 @@
     "code-search-ada-code-001": "r50k_base",
     # open source
     "gpt2": "gpt2",
+    "gpt-2": "gpt2",  # Maintains consistency with gpt-4
 }