`map()` stops every 1000 steps
### Describe the bug

I am passing the following `prepare_dataset` function to `Dataset.map` (code is inspired from [here](https://github.com/huggingface/community-events/blob/main/whisper-fine-tuning-event/run_speech_recognition_seq2seq_streaming.py#L454))
```python3
def prepare_dataset(batch):
    # load and resample audio data from 48 to 16kHz
    audio = batch["audio"]

    # compute log-Mel input features from input audio array 
    batch["input_features"] = feature_extractor(audio["array"], sampling_rate=audio["sampling_rate"]).input_features[0]

    # encode target text to label ids 
    batch["labels"] = tokenizer(batch[text_column]).input_ids
    return batch
...
train_ds = train_ds.map(prepare_dataset)
```

Here is the exact code I am running https://github.com/bayartsogt-ya/whisper-multiple-hf-datasets/blob/main/train.py#L70-L71

It starts using all the cores (I am not sure why because I did not pass `num_proc`)
then progress bar stops at every 1k steps. (starts using a single core)
then come back to using all the cores again.

link to [screen record](https://youtu.be/jPQpQQGp6Gc)

Can someone explain this process and maybe provide a way to improve this pipeline? cc: @lhoestq 

### Steps to reproduce the bug

1. load the dataset
2. create a Whisper processor
3. create a `prepare_dataset` function
4. pass the function to `dataset.map(prepare_dataset)`

### Expected behavior

- Use a single core per a function
- not to stop at some point?

### Environment info

- `datasets` version: 2.7.1.dev0
- Platform: Linux-5.4.0-109-generic-x86_64-with-glibc2.27
- Python version: 3.8.10
- PyArrow version: 10.0.1
- Pandas version: 1.5.2
Hi !

> It starts using all the cores (I am not sure why because I did not pass num_proc)

The tokenizer uses Rust code that is multithreaded. And maybe the `feature_extractor` might run some things in parallel as well - but I'm not super familiar with its internals.

> then progress bar stops at every 1k steps. (starts using a single core)

Every 1000 examples we flush the processed examples to disk. It is this way because Arrow is a columnar format: you must write data chunk by chunk. The processing in on hold while writing right now - maybe this can be improved in the future. Hi @lhoestq 
Thanks for the explanation! it was so helpful! Let me check why `feature_extractor` is running on multiple cpus.