Multithreaded downloads

...for faster dataset download when there are many many small files (e.g. imagefolder, audiofolder)

### Behcnmark

for example on [lhoestq/tmp-images-writer_batch_size](https://hf.co/datasets/lhoestq/tmp-images-writer_batch_size) (128 images)

|    | duration of the download step in `load_dataset()` |
|--| ----------------------------------------------------------------------|
| Before | 58s |
| Now | 3s |

This should fix issues with the Dataset Viewer taking too much time to show up for imagefolder/audiofolder datasets.

### Implementation details

The main change is in the `DownloadManager`:

```diff
- download_func = partial(self._download, download_config=download_config)
+ download_func = partial(self._download_batched, download_config=download_config)
downloaded_path_or_paths = map_nested(
    download_func,
    url_or_urls,
    map_tuple=True,
    num_proc=download_config.num_proc,
    desc="Downloading data files",
+   batched=True,
+   batch_size=-1,
)
```

and `_download_batched` is a multithreaded function.

I only enable multithreading if there are more than 16 files and files are small though, otherwise the progress bar that counts the number of downloaded files is not fluid (updating when a big batch of big files are done downloading). To do so I simply check if the first file is smaller than 20MB.

I also had to tweak `map_nested` to support batching. In particular it slices the data correctly if the user also enables multiprocessing.

Comments:
The docs for this PR live [here](https://moon-ci-docs.huggingface.co/docs/datasets/pr_6794). All of your documentation changes will be reflected on that endpoint. The docs are available until 30 days after the last update.

CI is failing because of the missing parquet export of one test dataset, PR to fix this at https://github.com/huggingface/dataset-viewer/pull/2689

I took your comments into account :) lmk what you think @mariosasko 

<details>
<summary>Show benchmarks</summary>

PyArrow==8.0.0

<details>
<summary>Show updated benchmarks!</summary>

### Benchmark: benchmark_array_xd.json

| metric | read_batch_formatted_as_numpy after write_array2d | read_batch_formatted_as_numpy after write_flattened_sequence | read_batch_formatted_as_numpy after write_nested_sequence | read_batch_unformated after write_array2d | read_batch_unformated after write_flattened_sequence | read_batch_unformated after write_nested_sequence | read_col_formatted_as_numpy after write_array2d | read_col_formatted_as_numpy after write_flattened_sequence | read_col_formatted_as_numpy after write_nested_sequence | read_col_unformated after write_array2d | read_col_unformated after write_flattened_sequence | read_col_unformated after write_nested_sequence | read_formatted_as_numpy after write_array2d | read_formatted_as_numpy after write_flattened_sequence | read_formatted_as_numpy after write_nested_sequence | read_unformated after write_array2d | read_unformated after write_flattened_sequence | read_unformated after write_nested_sequence | write_array2d | write_flattened_sequence | write_nested_sequence |
|--------|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| new / old (diff) | 0.004956 / 0.011353 (-0.006397) | 0.003282 / 0.011008 (-0.007726) | 0.064028 / 0.038508 (0.025520) | 0.030420 / 0.023109 (0.007311) | 0.240097 / 0.275898 (-0.035801) | 0.266356 / 0.323480 (-0.057124) | 0.003116 / 0.007986 (-0.004869) | 0.002597 / 0.004328 (-0.001731) | 0.050230 / 0.004250 (0.045980) | 0.043864 / 0.037052 (0.006812) | 0.258711 / 0.258489 (0.000222) | 0.290816 / 0.293841 (-0.003025) | 0.027898 / 0.128546 (-0.100648) | 0.009941 / 0.075646 (-0.065705) | 0.208917 / 0.419271 (-0.210355) | 0.035891 / 0.043533 (-0.007642) | 0.253332 / 0.255139 (-0.001807) | 0.274300 / 0.283200 (-0.008900) | 0.019466 / 0.141683 (-0.122217) | 1.133896 / 1.452155 (-0.318259) | 1.178130 / 1.492716 (-0.314586) |

### Benchmark: benchmark_getitem\_100B.json

| metric | get_batch_of\_1024\_random_rows | get_batch_of\_1024\_rows | get_first_row | get_last_row |
|--------|---|---|---|---|
| new / old (diff) | 0.091093 / 0.018006 (0.073087) | 0.293632 / 0.000490 (0.293142) | 0.000216 / 0.000200 (0.000016) | 0.000042 / 0.000054 (-0.000013) |

### Benchmark: benchmark_indices_mapping.json

| metric | select | shard | shuffle | sort | train_test_split |
|--------|---|---|---|---|---|
| new / old (diff) | 0.017722 / 0.037411 (-0.019689) | 0.060241 / 0.014526 (0.045715) | 0.072024 / 0.176557 (-0.104533) | 0.118521 / 0.737135 (-0.618615) | 0.071107 / 0.296338 (-0.225232) |

### Benchmark: benchmark_iterating.json

| metric | read 5000 | read 50000 | read_batch 50000 10 | read_batch 50000 100 | read_batch 50000 1000 | read_formatted numpy 5000 | read_formatted pandas 5000 | read_formatted tensorflow 5000 | read_formatted torch 5000 | read_formatted_batch numpy 5000 10 | read_formatted_batch numpy 5000 1000 | shuffled read 5000 | shuffled read 50000 | shuffled read_batch 50000 10 | shuffled read_batch 50000 100 | shuffled read_batch 50000 1000 | shuffled read_formatted numpy 5000 | shuffled read_formatted_batch numpy 5000 10 | shuffled read_formatted_batch numpy 5000 1000 |
|--------|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| new / old (diff) | 0.280950 / 0.215209 (0.065741) | 2.781361 / 2.077655 (0.703706) | 1.477949 / 1.504120 (-0.026171) | 1.356388 / 1.541195 (-0.184807) | 1.361808 / 1.468490 (-0.106682) | 0.565499 / 4.584777 (-4.019278) | 2.389206 / 3.745712 (-1.356506) | 2.712782 / 5.269862 (-2.557079) | 1.701402 / 4.565676 (-2.864274) | 0.063619 / 0.424275 (-0.360656) | 0.005321 / 0.007607 (-0.002286) | 0.336783 / 0.226044 (0.110739) | 3.299628 / 2.268929 (1.030699) | 1.794686 / 55.444624 (-53.649939) | 1.504207 / 6.876477 (-5.372270) | 1.524637 / 2.142072 (-0.617436) | 0.642833 / 4.805227 (-4.162395) | 0.117808 / 6.500664 (-6.382856) | 0.041539 / 0.075469 (-0.033930) |

### Benchmark: benchmark_map_filter.json

| metric | filter | map fast-tokenizer batched | map identity | map identity batched | map no-op batched | map no-op batched numpy | map no-op batched pandas | map no-op batched pytorch | map no-op batched tensorflow |
|--------|---|---|---|---|---|---|---|---|---|
| new / old (diff) | 0.960193 / 1.841788 (-0.881595) | 11.229147 / 8.074308 (3.154839) | 9.380653 / 10.191392 (-0.810739) | 0.137184 / 0.680424 (-0.543240) | 0.013399 / 0.534201 (-0.520802) | 0.314904 / 0.579283 (-0.264379) | 0.262539 / 0.434364 (-0.171825) | 0.354007 / 0.540337 (-0.186331) | 0.451698 / 1.386936 (-0.935238) |

</details>
PyArrow==latest

<details>
<summary>Show updated benchmarks!</summary>

### Benchmark: benchmark_array_xd.json

| metric | read_batch_formatted_as_numpy after write_array2d | read_batch_formatted_as_numpy after write_flattened_sequence | read_batch_formatted_as_numpy after write_nested_sequence | read_batch_unformated after write_array2d | read_batch_unformated after write_flattened_sequence | read_batch_unformated after write_nested_sequence | read_col_formatted_as_numpy after write_array2d | read_col_formatted_as_numpy after write_flattened_sequence | read_col_formatted_as_numpy after write_nested_sequence | read_col_unformated after write_array2d | read_col_unformated after write_flattened_sequence | read_col_unformated after write_nested_sequence | read_formatted_as_numpy after write_array2d | read_formatted_as_numpy after write_flattened_sequence | read_formatted_as_numpy after write_nested_sequence | read_unformated after write_array2d | read_unformated after write_flattened_sequence | read_unformated after write_nested_sequence | write_array2d | write_flattened_sequence | write_nested_sequence |
|--------|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| new / old (diff) | 0.005207 / 0.011353 (-0.006146) | 0.003660 / 0.011008 (-0.007348) | 0.049931 / 0.038508 (0.011423) | 0.030918 / 0.023109 (0.007809) | 0.271243 / 0.275898 (-0.004655) | 0.295706 / 0.323480 (-0.027774) | 0.004106 / 0.007986 (-0.003879) | 0.002750 / 0.004328 (-0.001578) | 0.048337 / 0.004250 (0.044086) | 0.039944 / 0.037052 (0.002892) | 0.284013 / 0.258489 (0.025524) | 0.306827 / 0.293841 (0.012987) | 0.029183 / 0.128546 (-0.099363) | 0.010033 / 0.075646 (-0.065613) | 0.058126 / 0.419271 (-0.361146) | 0.032427 / 0.043533 (-0.011106) | 0.276471 / 0.255139 (0.021332) | 0.288428 / 0.283200 (0.005229) | 0.017549 / 0.141683 (-0.124134) | 1.142361 / 1.452155 (-0.309793) | 1.184514 / 1.492716 (-0.308202) |

### Benchmark: benchmark_getitem\_100B.json

| metric | get_batch_of\_1024\_random_rows | get_batch_of\_1024\_rows | get_first_row | get_last_row |
|--------|---|---|---|---|
| new / old (diff) | 0.090350 / 0.018006 (0.072344) | 0.292511 / 0.000490 (0.292021) | 0.000215 / 0.000200 (0.000015) | 0.000041 / 0.000054 (-0.000013) |

### Benchmark: benchmark_indices_mapping.json

| metric | select | shard | shuffle | sort | train_test_split |
|--------|---|---|---|---|---|
| new / old (diff) | 0.021572 / 0.037411 (-0.015840) | 0.074310 / 0.014526 (0.059784) | 0.086102 / 0.176557 (-0.090455) | 0.123507 / 0.737135 (-0.613629) | 0.087397 / 0.296338 (-0.208941) |

### Benchmark: benchmark_iterating.json

| metric | read 5000 | read 50000 | read_batch 50000 10 | read_batch 50000 100 | read_batch 50000 1000 | read_formatted numpy 5000 | read_formatted pandas 5000 | read_formatted tensorflow 5000 | read_formatted torch 5000 | read_formatted_batch numpy 5000 10 | read_formatted_batch numpy 5000 1000 | shuffled read 5000 | shuffled read 50000 | shuffled read_batch 50000 10 | shuffled read_batch 50000 100 | shuffled read_batch 50000 1000 | shuffled read_formatted numpy 5000 | shuffled read_formatted_batch numpy 5000 10 | shuffled read_formatted_batch numpy 5000 1000 |
|--------|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| new / old (diff) | 0.294038 / 0.215209 (0.078829) | 2.889662 / 2.077655 (0.812007) | 1.591775 / 1.504120 (0.087655) | 1.468815 / 1.541195 (-0.072379) | 1.470226 / 1.468490 (0.001736) | 0.574557 / 4.584777 (-4.010220) | 2.481377 / 3.745712 (-1.264335) | 2.763368 / 5.269862 (-2.506493) | 1.713707 / 4.565676 (-2.851969) | 0.064158 / 0.424275 (-0.360117) | 0.005553 / 0.007607 (-0.002054) | 0.353480 / 0.226044 (0.127436) | 3.447689 / 2.268929 (1.178760) | 1.975802 / 55.444624 (-53.468822) | 1.673561 / 6.876477 (-5.202915) | 1.637212 / 2.142072 (-0.504860) | 0.640667 / 4.805227 (-4.164560) | 0.114618 / 6.500664 (-6.386046) | 0.038912 / 0.075469 (-0.036557) |

### Benchmark: benchmark_map_filter.json

| metric | filter | map fast-tokenizer batched | map identity | map identity batched | map no-op batched | map no-op batched numpy | map no-op batched pandas | map no-op batched pytorch | map no-op batched tensorflow |
|--------|---|---|---|---|---|---|---|---|---|
| new / old (diff) | 1.007581 / 1.841788 (-0.834207) | 11.874250 / 8.074308 (3.799942) | 10.312692 / 10.191392 (0.121300) | 0.142705 / 0.680424 (-0.537719) | 0.015438 / 0.534201 (-0.518763) | 0.285919 / 0.579283 (-0.293364) | 0.278223 / 0.434364 (-0.156141) | 0.323806 / 0.540337 (-0.216531) | 0.415007 / 1.386936 (-0.971929) |

</details>
</details>

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