Implement a decode method in transformers.BasicTokenizer
### Feature request

Transformers has provided a nice BasicTokenizer for basic tokenizing when we don't need BPE tokenizers. For data processing (like data format converting), it is better to offer a decode method for basic use.

### Motivation

When doing data format converting in some data processing problems, we usually meet the requirement to recover a list of tokens into continuous, readable text.

### Your contribution

None.
@jiangwy99 The BasicTokenizer class will perform simple string processing e.g. splitting on white spaces. However it doesn't encode the tokens to ids, and so doesn't have a corresponding `decode` method. 

cc @ArthurZucker <|||||>@amyeroberts BasicTokenizer has implemented a tokenize function, which converts text into a list of tokens. What I want is a de-tokenize, which converts a list of tokenized tokens into the original text, serving as a dual operation of BasicTokenizer.tokenize<|||||>@jiangwy99  The BasicTokenizer is just that, a very simple class used for doing basic preprocessing of strings: puncutation splitting, making lower case etc. Adding this functionality is beyond the scope of the current class. 

If it's something you're still interested in, you're welcome to make a fork of the repo with an implementation and share it here. The forum is [a good place](https://discuss.huggingface.co/) to discuss questions about implementation details.  Note: it won't be possible to always exactly recover the original input if `do_lower_case=True`, and special handling of spacing and capitalization with punctuation would be required. 