feat(custom_llm.py): initial working commit for writing your own custom LLM handler

Fixes https://github.com/BerriAI/litellm/issues/4675

 Also Addresses https://github.com/BerriAI/litellm/discussions/4677

litellm/tests/test_custom_llm.py

@@ -0,0 +1,63 @@
+# What is this?
+## Unit tests for the CustomLLM class
+
+
+import asyncio
+import os
+import sys
+import time
+import traceback
+
+import openai
+import pytest
+
+sys.path.insert(
+    0, os.path.abspath("../..")
+)  # Adds the parent directory to the system path
+import os
+from collections import defaultdict
+from concurrent.futures import ThreadPoolExecutor
+from unittest.mock import AsyncMock, MagicMock, patch
+
+import httpx
+from dotenv import load_dotenv
+
+import litellm
+from litellm import CustomLLM, completion, get_llm_provider
+
+
+class MyCustomLLM(CustomLLM):
+    def completion(self, *args, **kwargs) -> litellm.ModelResponse:
+        return litellm.completion(
+            model="gpt-3.5-turbo",
+            messages=[{"role": "user", "content": "Hello world"}],
+            mock_response="Hi!",
+        )  # type: ignore
+
+
+def test_get_llm_provider():
+    from litellm.utils import custom_llm_setup
+
+    my_custom_llm = MyCustomLLM()
+    litellm.custom_provider_map = [
+        {"provider": "custom_llm", "custom_handler": my_custom_llm}
+    ]
+
+    custom_llm_setup()
+
+    model, provider, _, _ = get_llm_provider(model="custom_llm/my-fake-model")
+
+    assert provider == "custom_llm"
+
+
+def test_simple_completion():
+    my_custom_llm = MyCustomLLM()
+    litellm.custom_provider_map = [
+        {"provider": "custom_llm", "custom_handler": my_custom_llm}
+    ]
+    resp = completion(
+        model="custom_llm/my-fake-model",
+        messages=[{"role": "user", "content": "Hello world!"}],
+    )
+
+    assert resp.choices[0].message.content == "Hi!"