community[minor]: Add support for Perplexity chat model(#17024)

- **Description:** This PR adds support for [Perplexity AI
APIs](https://blog.perplexity.ai/blog/introducing-pplx-api).
  - **Issues:** None
  - **Dependencies:** None
  - **Twitter handle:** [@atherfawaz](https://twitter.com/AtherFawaz)

---------

Co-authored-by: Bagatur <baskaryan@gmail.com>
Co-authored-by: Erick Friis <erick@langchain.dev>

libs/community/tests/unit_tests/chat_models/test_imports.py

@@ -41,6 +41,7 @@ EXPECTED_ALL = [
     "GPTRouter",
     "ChatYuan2",
     "ChatZhipuAI",
+    "ChatPerplexity",
     "ChatKinetica",
 ]
 

libs/community/tests/unit_tests/chat_models/test_perplexity.py

@@ -0,0 +1,30 @@
+"""Test Perplexity Chat API wrapper."""
+import os
+
+import pytest
+
+from langchain_community.chat_models import ChatPerplexity
+
+os.environ["PPLX_API_KEY"] = "foo"
+
+
+@pytest.mark.requires("openai")
+def test_perplexity_model_name_param() -> None:
+    llm = ChatPerplexity(model="foo")
+    assert llm.model == "foo"
+
+
+@pytest.mark.requires("openai")
+def test_perplexity_model_kwargs() -> None:
+    llm = ChatPerplexity(model="test", model_kwargs={"foo": "bar"})
+    assert llm.model_kwargs == {"foo": "bar"}
+
+
+@pytest.mark.requires("openai")
+def test_perplexity_initialization() -> None:
+    """Test perplexity initialization."""
+    # Verify that chat perplexity can be initialized using a secret key provided
+    # as a parameter rather than an environment variable.
+    ChatPerplexity(
+        model="test", perplexity_api_key="test", temperature=0.7, verbose=True
+    )