add validation for stream=False and yield usage

Author: aniketmaurya

src/litserve/loops/base.py

@@ -264,6 +264,29 @@ def pre_setup(self, lit_api: LitAPI, spec: Optional[LitSpec]):
             return
 
         original = lit_api.unbatch.__code__ is LitAPI.unbatch.__code__
+        if not lit_api.stream and any([
+                inspect.isgeneratorfunction(lit_api.predict),
+                inspect.isgeneratorfunction(lit_api.encode_response),
+                (original or inspect.isgeneratorfunction(lit_api.unbatch)),
+            ]):
+            raise ValueError(
+                """When `stream=False`, `lit_api.predict`, `lit_api.encode_response` and `lit_api.unbatch` must not be
+                generator functions.
+
+                Correct example:
+
+                    def predict(self, inputs):
+                        ...
+                        return {"output": output}
+
+                Incorrect example:
+
+                    def predict(self, inputs):
+                        ...
+                        for i in range(max_token_length):
+                            yield prediction
+                """
+            )
         if (
             lit_api.stream
             and lit_api.max_batch_size > 1