update(llms/TGI): Allow None as temperature value (#10212)

Text Generation Inference's client permits the use of a None temperature
as seen
[here](033230ae66/clients/python/text_generation/client.py (L71C9-L71C20)).
While I haved dived into TGI's server code and don't know about the
implications of using None as a temperature setting, I think we should
grant users the option to pass None as a temperature parameter to TGI.

libs/langchain/langchain/llms/huggingface_text_gen_inference.py

@@ -65,7 +65,7 @@ class HuggingFaceTextGenInference(LLM):
     typical_p: Optional[float] = 0.95
     """Typical Decoding mass. See [Typical Decoding for Natural Language
     Generation](https://arxiv.org/abs/2202.00666) for more information."""
-    temperature: float = 0.8
+    temperature: Optional[float] = 0.8
     """The value used to module the logits distribution."""
     repetition_penalty: Optional[float] = None
     """The parameter for repetition penalty. 1.0 means no penalty.