add logger (#529)

As talking #519, I made 2 PRs.

this is the first PR for adding a logger.

I am concerned about the following two points and would appreciate your
opinion.

1. Since the logger is not formatted, the statement itself is output
like a print statement, and I thought it was difficult to understand
that it was a warning, so I put WARNING! at the beginning of the warning
statement. After the logger formatting is done properly, the word
WARNING can be repeated.
2. Statement `Please confirm that {field_name} is what you intended.`
can be replaced like `If {field_name} is intended parameters, enter it
to model_kwargs`
thank you!

Yongtae

langchain/llms/openai.py

@@ -1,4 +1,5 @@
 """Wrapper around OpenAI APIs."""
+import logging
 import sys
 from typing import Any, Dict, Generator, List, Mapping, Optional, Tuple, Union
 
@@ -8,6 +9,8 @@ from langchain.llms.base import BaseLLM
 from langchain.schema import Generation, LLMResult
 from langchain.utils import get_from_dict_or_env
 
+logger = logging.getLogger(__name__)
+
 
 class BaseOpenAI(BaseLLM, BaseModel):
     """Wrapper around OpenAI large language models.
@@ -67,6 +70,11 @@ class BaseOpenAI(BaseLLM, BaseModel):
             if field_name not in all_required_field_names:
                 if field_name in extra:
                     raise ValueError(f"Found {field_name} supplied twice.")
+                logger.warning(
+                    f"""WARNING! {field_name} is not default parameter.
+                    {field_name} was transfered to model_kwargs.
+                    Please confirm that {field_name} is what you intended."""
+                )
                 extra[field_name] = values.pop(field_name)
         values["model_kwargs"] = extra
         return values