Incorrect type hint of "exponential_decay_length_penalty" in function "generate"
Hi, 
   Please check below line, 
https://github.com/huggingface/transformers/blob/7e7f62bfa72ca03e9f16285dad182f7c57cd8cab/src/transformers/generation_utils.py#L956
   According to doc string, "exponential_decay_length_penalty (`tuple(int, float)`, *optional*, defaults to `model.config.exponential_decay_length_penalty`):" (https://github.com/huggingface/transformers/blob/main/src/transformers/generation_utils.py#L1114)

the correct type hint should be "Optional[Tuple[int, float]]", the number of tuple elements should be 2, and must be "int" in position 0, and "float" in position 1. 