Add EncodedInput as an alternative input type of _batch_encode_plus in tokenization_utils_fast
# 🚀 Feature request

The [_batch_encode_plus](https://github.com/huggingface/transformers/blob/6989264963d1f8871404889243d2f15de198ee42/src/transformers/tokenization_utils.py#L483) function in [tokenization_utils](https://github.com/huggingface/transformers/blob/master/src/transformers/tokenization_utils.py) supports to use EncodedInput and EncodedInputPair as inputs. I hope the fast tokenizer can also support that in [tokenization_utils_fast](https://github.com/huggingface/transformers/blob/master/src/transformers/tokenization_utils_fast.py).

## Motivation

It would be more flexible to add EncodedInput and EncodedInputPair as alternative input types for the default fast tokenizer, meanwhile this feature has been implemented in the original tokenizer.

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