patch: remove usage of llm, chat model __call__ (#20788)

- `llm(prompt)` -> `llm.invoke(prompt)`
- `llm(prompt=prompt` -> `llm.invoke(prompt)` (same with `messages=`)
- `llm(prompt, callbacks=callbacks)` -> `llm.invoke(prompt,
config={"callbacks": callbacks})`
- `llm(prompt, **kwargs)` -> `llm.invoke(prompt, **kwargs)`

libs/community/langchain_community/llms/textgen.py

@@ -199,7 +199,7 @@ class TextGen(LLM):
 
                 from langchain_community.llms import TextGen
                 llm = TextGen(model_url="http://localhost:5000")
-                llm("Write a story about llamas.")
+                llm.invoke("Write a story about llamas.")
         """
         if self.streaming:
             combined_text_output = ""
@@ -245,7 +245,7 @@ class TextGen(LLM):
 
                 from langchain_community.llms import TextGen
                 llm = TextGen(model_url="http://localhost:5000")
-                llm("Write a story about llamas.")
+                llm.invoke("Write a story about llamas.")
         """
         if self.streaming:
             combined_text_output = ""