add openai logit bias (#553)

Add
[`logit_bias`](https://beta.openai.com/docs/api-reference/completions/create#completions/create-logit_bias)
params to OpenAI

See [here](https://beta.openai.com/tokenizer) for the tokenizer.

NB: I see that others (like Cohere) have the same parameter, but since I
don't have an access to it, I don't want to make a mistake.

---

Just to make sure the default "{}" works for openai:
```
from langchain.llms import OpenAI

OPENAI_API_KEY="XXX"

llm = OpenAI(openai_api_key=OPENAI_API_KEY)
llm.generate('Write "test":')

llm = OpenAI(openai_api_key=OPENAI_API_KEY, logit_bias={'9288': -100, '1332': -100, '14402': -100, '6208': -100})
llm.generate('Write "test":')
```

langchain/llms/openai.py

@@ -54,6 +54,8 @@ class BaseOpenAI(BaseLLM, BaseModel):
     """Batch size to use when passing multiple documents to generate."""
     request_timeout: Optional[Union[float, Tuple[float, float]]] = None
     """Timeout for requests to OpenAI completion API. Default is 600 seconds."""
+    logit_bias: Optional[Dict[str, float]] = Field(default_factory=dict)
+    """Adjust the probability of specific tokens being generated."""
 
     class Config:
         """Configuration for this pydantic object."""
@@ -109,6 +111,7 @@ class BaseOpenAI(BaseLLM, BaseModel):
             "n": self.n,
             "best_of": self.best_of,
             "request_timeout": self.request_timeout,
+            "logit_bias": self.logit_bias,
         }
         return {**normal_params, **self.model_kwargs}