community[minor]: Integrating GPTRouter (#14900)

**Description:** Adding a langchain integration for
[GPTRouter](https://gpt-router.writesonic.com/) 🚀 ,
 **Tag maintainer:** @Gupta-Anubhav12 @samanyougarg @sirjan-ws-ext  
 **Twitter handle:** [@SamanyouGarg](https://twitter.com/SamanyouGarg)
 
Integration Tests Passing:
<img width="1137" alt="Screenshot 2023-12-19 at 5 45 31 PM"
src="https://github.com/Writesonic/langchain/assets/151817113/4a59df9a-ee30-47aa-9df9-b8c4eeb9dc76">

libs/community/tests/integration_tests/chat_models/test_gpt_router.py

@@ -0,0 +1,84 @@
+"""Test GPTRouter API wrapper."""
+from typing import List
+
+import pytest
+from langchain_core.callbacks import (
+    CallbackManager,
+)
+from langchain_core.messages import AIMessage, BaseMessage, HumanMessage
+from langchain_core.outputs import ChatGeneration, LLMResult
+
+from langchain_community.chat_models.gpt_router import ChatGPTRouter, GPTRouterModel
+from tests.unit_tests.callbacks.fake_callback_handler import FakeCallbackHandler
+
+
+def test_gpt_router_call() -> None:
+    """Test valid call to GPTRouter."""
+    anthropic_claude = GPTRouterModel(
+        name="claude-instant-1.2", provider_name="anthropic"
+    )
+    chat = ChatGPTRouter(models_priority_list=[anthropic_claude])
+    message = HumanMessage(content="Hello World")
+    response = chat([message])
+    assert isinstance(response, AIMessage)
+    assert isinstance(response.content, str)
+
+
+def test_gpt_router_call_incorrect_model() -> None:
+    """Test invalid modelName"""
+    anthropic_claude = GPTRouterModel(
+        name="model_does_not_exist", provider_name="anthropic"
+    )
+    chat = ChatGPTRouter(models_priority_list=[anthropic_claude])
+    message = HumanMessage(content="Hello World")
+    with pytest.raises(Exception):
+        chat([message])
+
+
+def test_gpt_router_generate() -> None:
+    """Test generate method of GPTRouter."""
+    anthropic_claude = GPTRouterModel(
+        name="claude-instant-1.2", provider_name="anthropic"
+    )
+    chat = ChatGPTRouter(models_priority_list=[anthropic_claude])
+    chat_messages: List[List[BaseMessage]] = [
+        [HumanMessage(content="If (5 + x = 18), what is x?")]
+    ]
+    messages_copy = [messages.copy() for messages in chat_messages]
+    result: LLMResult = chat.generate(chat_messages)
+    assert isinstance(result, LLMResult)
+    for response in result.generations[0]:
+        assert isinstance(response, ChatGeneration)
+        assert isinstance(response.text, str)
+        assert response.text == response.message.content
+    assert chat_messages == messages_copy
+
+
+def test_gpt_router_streaming() -> None:
+    """Test streaming tokens from GPTRouter."""
+    anthropic_claude = GPTRouterModel(
+        name="claude-instant-1.2", provider_name="anthropic"
+    )
+    chat = ChatGPTRouter(models_priority_list=[anthropic_claude], streaming=True)
+    message = HumanMessage(content="Hello")
+    response = chat([message])
+    assert isinstance(response, AIMessage)
+    assert isinstance(response.content, str)
+
+
+def test_gpt_router_streaming_callback() -> None:
+    """Test that streaming correctly invokes on_llm_new_token callback."""
+    callback_handler = FakeCallbackHandler()
+    callback_manager = CallbackManager([callback_handler])
+    anthropic_claude = GPTRouterModel(
+        name="claude-instant-1.2", provider_name="anthropic"
+    )
+    chat = ChatGPTRouter(
+        models_priority_list=[anthropic_claude],
+        streaming=True,
+        callback_manager=callback_manager,
+        verbose=True,
+    )
+    message = HumanMessage(content="Write me a 5 line poem.")
+    chat([message])
+    assert callback_handler.llm_streams > 1