langchain[patch]: init_chat_model provider in model string (#28367)

```python
llm = init_chat_model("openai:gpt-4o")
```

libs/langchain/langchain/chat_models/base.py

@@ -328,13 +328,7 @@ class GetPopulation(BaseModel):
 def _init_chat_model_helper(
     model: str, *, model_provider: Optional[str] = None, **kwargs: Any
 ) -> BaseChatModel:
-    model_provider = model_provider or _attempt_infer_model_provider(model)
-    if not model_provider:
-        raise ValueError(
-            f"Unable to infer model provider for {model=}, please specify "
-            f"model_provider directly."
-        )
-    model_provider = model_provider.replace("-", "_").lower()
+    model, model_provider = _parse_model(model, model_provider)
     if model_provider == "openai":
         _check_pkg("langchain_openai")
         from langchain_openai import ChatOpenAI
@@ -461,6 +455,24 @@ def _attempt_infer_model_provider(model_name: str) -> Optional[str]:
         return None
 
 
+def _parse_model(model: str, model_provider: Optional[str]) -> Tuple[str, str]:
+    if (
+        not model_provider
+        and ":" in model
+        and model.split(":")[0] in _SUPPORTED_PROVIDERS
+    ):
+        model_provider = model.split(":")[0]
+        model = ":".join(model.split(":")[1:])
+    model_provider = model_provider or _attempt_infer_model_provider(model)
+    if not model_provider:
+        raise ValueError(
+            f"Unable to infer model provider for {model=}, please specify "
+            f"model_provider directly."
+        )
+    model_provider = model_provider.replace("-", "_").lower()
+    return model, model_provider
+
+
 def _check_pkg(pkg: str) -> None:
     if not util.find_spec(pkg):
         pkg_kebab = pkg.replace("_", "-")

libs/langchain/tests/unit_tests/chat_models/test_base.py

@@ -1,4 +1,5 @@
 import os
+from typing import Optional
 from unittest import mock
 
 import pytest
@@ -26,7 +27,6 @@ def test_all_imports() -> None:
     "langchain_openai",
     "langchain_anthropic",
     "langchain_fireworks",
-    "langchain_mistralai",
     "langchain_groq",
 )
 @pytest.mark.parametrize(
@@ -38,10 +38,14 @@ def test_all_imports() -> None:
         ("mixtral-8x7b-32768", "groq"),
     ],
 )
-def test_init_chat_model(model_name: str, model_provider: str) -> None:
-    _: BaseChatModel = init_chat_model(
+def test_init_chat_model(model_name: str, model_provider: Optional[str]) -> None:
+    llm1: BaseChatModel = init_chat_model(
         model_name, model_provider=model_provider, api_key="foo"
     )
+    llm2: BaseChatModel = init_chat_model(
+        f"{model_provider}:{model_name}", api_key="foo"
+    )
+    assert llm1.dict() == llm2.dict()
 
 
 def test_init_missing_dep() -> None: