Progress bars are not properly rendered in Jupyter notebook
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To add my experience when trying to debug this issue:

Seems like previously the workaround given [here](https://github.com/tqdm/tqdm/issues/485#issuecomment-473338308) worked around this issue. But with the latest version of jupyter/tqdm I still get terminal warnings that IPython tried to send a message from a forked process. Hi @mludv, thanks for the hint!!! :) 

We will definitely take it into account to try to fix this issue... It seems somehow related to `multiprocessing` and `tqdm`...
## Describe the bug
The progress bars are not Jupyter widgets; regular progress bars appear (like in a terminal).

## Steps to reproduce the bug
```python
ds.map(tokenize, num_proc=10)
```

## Expected results
Jupyter widgets displaying the progress bars.

## Actual results
Simple plane progress bars.

cc: Reported by @thomwolf 
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