experimental[minor]: adds mixtral wrapper (#17423)

**Description:** Adds a chat wrapper for Mixtral models using the
[prompt
template](https://huggingface.co/mistralai/Mixtral-8x7B-Instruct-v0.1#instruction-format).

---------

Co-authored-by: Bagatur <baskaryan@gmail.com>

libs/experimental/tests/unit_tests/chat_models/test_llm_wrapper_mixtral.py

@@ -0,0 +1,31 @@
+import pytest
+from langchain.schema import AIMessage, HumanMessage, SystemMessage
+
+from langchain_experimental.chat_models import Mixtral
+from tests.unit_tests.chat_models.test_llm_wrapper_llama2chat import FakeLLM
+
+
+@pytest.fixture
+def model() -> Mixtral:
+    return Mixtral(llm=FakeLLM())
+
+
+@pytest.fixture
+def model_cfg_sys_msg() -> Mixtral:
+    return Mixtral(llm=FakeLLM(), system_message=SystemMessage(content="sys-msg"))
+
+
+def test_prompt(model: Mixtral) -> None:
+    messages = [
+        SystemMessage(content="sys-msg"),
+        HumanMessage(content="usr-msg-1"),
+        AIMessage(content="ai-msg-1"),
+        HumanMessage(content="usr-msg-2"),
+    ]
+
+    actual = model.predict_messages(messages).content  # type: ignore
+    expected = (
+        "<s>[INST] sys-msg\nusr-msg-1 [/INST] ai-msg-1 </s> [INST] usr-msg-2 [/INST]"  # noqa: E501
+    )
+
+    assert actual == expected