Use `invoke` instead of `__call__` (#16369)
The following warning information will be displayed when i use
`llm(PROMPT)`:
```python
/Users/169/llama.cpp/venv/lib/python3.11/site-packages/langchain_core/_api/deprecation.py:117: LangChainDeprecationWarning: The function `__call__` was deprecated in LangChain 0.1.7 and will be removed in 0.2.0. Use invoke instead.
warn_deprecated(
```
So I changed to standard usage.
