community[fix]: Update Perplexity to pass parameters into API calls (#28421)

- [x] **PR title**: "package: description"
- Where "package" is whichever of langchain, community, core, etc. is
being modified. Use "docs: ..." for purely docs changes, "infra: ..."
for CI changes.
  - Example: "community: add foobar LLM"


- **Description:** I realized the invocation parameters were not being
passed into `_generate` so I added those in but then realized that the
parameters contained some old fields designed for an older openai client
which I removed. Parameters work fine now.
- **Issue:** Fixes #28229 
- **Dependencies:** No new dependencies.  
- **Twitter handle:** @arch_plane

- [x] **Lint and test**: Run `make format`, `make lint` and `make test`
from the root of the package(s) you've modified. See contribution
guidelines for more: https://python.langchain.com/docs/contributing/

Additional guidelines:
- Make sure optional dependencies are imported within a function.
- Please do not add dependencies to pyproject.toml files (even optional
ones) unless they are required for unit tests.
- Most PRs should not touch more than one package.
- Changes should be backwards compatible.
- If you are adding something to community, do not re-import it in
langchain.

If no one reviews your PR within a few days, please @-mention one of
baskaryan, efriis, eyurtsev, ccurme, vbarda, hwchase17.

Co-authored-by: Erick Friis <erick@langchain.dev>

libs/community/langchain_community/chat_models/perplexity.py

@@ -148,7 +148,6 @@ class ChatPerplexity(BaseChatModel):
     def _default_params(self) -> Dict[str, Any]:
         """Get the default parameters for calling PerplexityChat API."""
         return {
-            "request_timeout": self.request_timeout,
             "max_tokens": self.max_tokens,
             "stream": self.streaming,
             "temperature": self.temperature,
@@ -222,7 +221,7 @@ class ChatPerplexity(BaseChatModel):
         if stop:
             params["stop_sequences"] = stop
         stream_resp = self.client.chat.completions.create(
-            model=params["model"], messages=message_dicts, stream=True
+            messages=message_dicts, stream=True, **params
         )
         for chunk in stream_resp:
             if not isinstance(chunk, dict):
@@ -258,9 +257,7 @@ class ChatPerplexity(BaseChatModel):
                 return generate_from_stream(stream_iter)
         message_dicts, params = self._create_message_dicts(messages, stop)
         params = {**params, **kwargs}
-        response = self.client.chat.completions.create(
-            model=params["model"], messages=message_dicts
-        )
+        response = self.client.chat.completions.create(messages=message_dicts, **params)
         message = AIMessage(
             content=response.choices[0].message.content,
             additional_kwargs={"citations": response.citations},
@@ -271,8 +268,6 @@ class ChatPerplexity(BaseChatModel):
     def _invocation_params(self) -> Mapping[str, Any]:
         """Get the parameters used to invoke the model."""
         pplx_creds: Dict[str, Any] = {
-            "api_key": self.pplx_api_key,
-            "api_base": "https://api.perplexity.ai",
             "model": self.model,
         }
         return {**pplx_creds, **self._default_params}