Appending mapped dataset to list changes previous elements of a list
### System Info

Hi, 

I've been trying to tokenize a dataset with different tokenizers and store it, but in doing so, am running into a bug. The general idea is that appending to a list of datasets, seems to modify previous elements.

A code notebook is here: https://colab.research.google.com/drive/1ljMwBqzCe1fHffBcPP2py9IJMrocoNIU



### Who can help?

@ArthurZucker @younesbelkada 

### Information

- [ ] The official example scripts
- [X] My own modified scripts

### Tasks

- [ ] An officially supported task in the `examples` folder (such as GLUE/SQuAD, ...)
- [X] My own task or dataset (give details below)

### Reproduction

https://colab.research.google.com/drive/1ljMwBqzCe1fHffBcPP2py9IJMrocoNIU

### Expected behavior

Appending to the list of datasets shouldn't modify previous elements of that list.
update- the error seems to be in the use of the lambda function, no idea why<|||||>@surya-narayanan This isn't an issue specific to `datasets` - it's a Python behaviour due to binding of the `x` var when then `lambda` function is first defined:
https://docs.python.org/3/faq/programming.html#why-do-lambdas-defined-in-a-loop-with-different-values-all-return-the-same-result

For future issues experienced when using `datasets`, could you make sure to open them under the [datasets repo](https://github.com/huggingface/datasets)? <|||||>Great, thanks :) 