feat(router.py): enable pre-call checks

filter models outside of context window limits of a given message for a model group

 https://github.com/BerriAI/litellm/issues/872

litellm/tests/test_router.py

@@ -297,6 +297,55 @@ def test_router_azure_acompletion():
 
 # test_router_azure_acompletion()
 
+
+def test_router_context_window_check():
+    """
+    - Give a gpt-3.5-turbo model group with different context windows (4k vs. 16k)
+    - Send a 10k prompt
+    - Assert it works
+    """
+    from large_text import text
+    import os
+
+    litellm.set_verbose = False
+
+    print(f"len(text): {len(text)}")
+    try:
+        model_list = [
+            {
+                "model_name": "gpt-3.5-turbo",  # openai model name
+                "litellm_params": {  # params for litellm completion/embedding call
+                    "model": "azure/chatgpt-v-2",
+                    "api_key": os.getenv("AZURE_API_KEY"),
+                    "api_version": os.getenv("AZURE_API_VERSION"),
+                    "api_base": os.getenv("AZURE_API_BASE"),
+                },
+            },
+            {
+                "model_name": "gpt-3.5-turbo",  # openai model name
+                "litellm_params": {  # params for litellm completion/embedding call
+                    "model": "gpt-3.5-turbo-1106",
+                    "api_key": os.getenv("OPENAI_API_KEY"),
+                },
+            },
+        ]
+
+        router = Router(model_list=model_list, set_verbose=True, enable_pre_call_checks=True)  # type: ignore
+
+        response = router.completion(
+            model="gpt-3.5-turbo",
+            messages=[
+                {"role": "system", "content": text},
+                {"role": "user", "content": "Who was Alexander?"},
+            ],
+        )
+
+        print(f"response: {response}")
+        raise Exception("it worked!")
+    except Exception as e:
+        pytest.fail(f"Got unexpected exception on router! - {str(e)}")
+
+
 ### FUNCTION CALLING