[Feature] Calculate max_out_len without hard code for OpenAI model (#158)

* calulate max_out_len without hard code

* set default value

* update configs

* Update configs/eval_gpt3.5.py
Co-authored-by: Tong Gao <gaotongxiao@gmail.com>

---------
Co-authored-by: Tong Gao <gaotongxiao@gmail.com>

configs/eval_gpt3.5.py

@@ -24,7 +24,7 @@ models = [
         key='ENV',  # The key will be obtained from $OPENAI_API_KEY, but you can write down your key here as well
         meta_template=api_meta_template,
         query_per_second=1,
-        max_out_len=2048, max_seq_len=2048, batch_size=8),
+        max_out_len=2048, max_seq_len=4096, batch_size=8),
 ]
 
 infer = dict(

opencompass/models/openai_api.py

@@ -50,8 +50,8 @@ class OpenAI(BaseAPIModel):
     is_api: bool = True
 
     def __init__(self,
-                 path: str,
-                 max_seq_len: int = 2048,
+                 path: str = 'gpt-3.5-turbo',
+                 max_seq_len: int = 4096,
                  query_per_second: int = 1,
                  retry: int = 2,
                  key: Union[str, List[str]] = 'ENV',
@@ -146,7 +146,9 @@ class OpenAI(BaseAPIModel):
                 messages.append(msg)
 
         # max num token for gpt-3.5-turbo is 4097
-        max_out_len = min(max_out_len, 4000 - self.get_token_len(str(input)))
+        max_out_len = min(
+            max_out_len,
+            self.max_seq_len - 50 - self.get_token_len(str(input)))
         if max_out_len <= 0:
             return ''