Add parallel module using joblib for Spark

Discussion in https://github.com/huggingface/datasets/issues/5798

Hi @lhoestq, I added the `parallel` part according to the discussion we had. Could you take a look to see if this is aligned with your proposal?

Meanwhile I'm working on adding a `parallel_backend` parameter to `load_datasets` so that it can be used like:
```python
with parallel_backend('spark', steps=['downloading']) as backend:
  ds = load_dataset(..., parallel_backend=backend)
```
where `parallel_backend` is a `ParallelBackend` class.

_The documentation is not available anymore as the PR was closed or merged._

@lhoestq Thanks for the comments!
With your suggestion, no changes made to `load_dataset` and I validated that downloading with spark is working now  with this:
```py
with parallel_backend('spark', steps=["download"]):
      dataset = load_dataset(..., num_proc=2)
```

@lhoestq Can a maintainer help trigger the tests again?
> One idea is to decorate the download method to set the current global step to "download", and then only use joblib if the current step is one of the steps provided in parallel_backend.

Yes I think this is doable in a subsequent PR.
For throwing `NotImplementedError` I also think it can be done in a subsequent PR, because I'm not sure if `Dataset.map` is the only function that a user would expect to run using `with parallel_backend`.

Just triggered the tests :)

> Yes I think this is doable in a subsequent PR.
For throwing NotImplementedError I also think it can be done in a subsequent PR, because I'm not sure if Dataset.map is the only function that a user would expect to run using with parallel_backend.

I think any Dataset method that has a `num_proc` argument: Dataset.map (the other methods like filter or cast or based on map), and later we can see for the to_xxx methods (to_csv, to_parquet, etc.)

Hi maintainers, I've just addressed most of the comments, please take another look, thank you.

<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) | 0.008422 / 0.011353 (-0.002931) | 0.005658 / 0.011008 (-0.005350) | 0.135372 / 0.038508 (0.096864) | 0.044766 / 0.023109 (0.021657) | 0.417876 / 0.275898 (0.141978) | 0.462785 / 0.323480 (0.139305) | 0.005485 / 0.007986 (-0.002501) | 0.005640 / 0.004328 (0.001311) | 0.105020 / 0.004250 (0.100770) | 0.049114 / 0.037052 (0.012062) | 0.490450 / 0.258489 (0.231961) | 0.467693 / 0.293841 (0.173852) | 0.050929 / 0.128546 (-0.077617) | 0.014644 / 0.075646 (-0.061002) | 0.452373 / 0.419271 (0.033101) | 0.074897 / 0.043533 (0.031364) | 0.425816 / 0.255139 (0.170677) | 0.420415 / 0.283200 (0.137215) | 0.134121 / 0.141683 (-0.007561) | 1.927744 / 1.452155 (0.475589) | 2.014417 / 1.492716 (0.521701) |

### 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) | 0.254811 / 0.018006 (0.236805) | 0.550011 / 0.000490 (0.549521) | 0.004913 / 0.000200 (0.004714) | 0.000117 / 0.000054 (0.000062) |

### Benchmark: benchmark_indices_mapping.json

| metric | select | shard | shuffle | sort | train_test_split |
|--------|---|---|---|---|---|
| new / old (diff) | 0.032644 / 0.037411 (-0.004768) | 0.135672 / 0.014526 (0.121146) | 0.158984 / 0.176557 (-0.017572) | 0.218267 / 0.737135 (-0.518869) | 0.150348 / 0.296338 (-0.145991) |

### 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) | 0.625723 / 0.215209 (0.410514) | 6.247559 / 2.077655 (4.169905) | 2.626785 / 1.504120 (1.122666) | 2.195224 / 1.541195 (0.654030) | 2.232140 / 1.468490 (0.763650) | 0.943082 / 4.584777 (-3.641695) | 5.799262 / 3.745712 (2.053550) | 2.849411 / 5.269862 (-2.420450) | 1.744160 / 4.565676 (-2.821516) | 0.119056 / 0.424275 (-0.305219) | 0.014233 / 0.007607 (0.006626) | 0.795238 / 0.226044 (0.569194) | 7.569586 / 2.268929 (5.300657) | 3.179481 / 55.444624 (-52.265143) | 2.519772 / 6.876477 (-4.356704) | 2.714570 / 2.142072 (0.572498) | 1.107197 / 4.805227 (-3.698030) | 0.229986 / 6.500664 (-6.270678) | 0.087993 / 0.075469 (0.012524) |

### 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) | 1.535610 / 1.841788 (-0.306178) | 18.639369 / 8.074308 (10.565061) | 21.081844 / 10.191392 (10.890452) | 0.253247 / 0.680424 (-0.427177) | 0.026711 / 0.534201 (-0.507490) | 0.503790 / 0.579283 (-0.075493) | 0.600124 / 0.434364 (0.165760) | 0.617944 / 0.540337 (0.077607) | 0.766947 / 1.386936 (-0.619989) |

</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) | 0.007885 / 0.011353 (-0.003468) | 0.004761 / 0.011008 (-0.006248) | 0.097995 / 0.038508 (0.059487) | 0.033624 / 0.023109 (0.010515) | 0.504307 / 0.275898 (0.228409) | 0.534803 / 0.323480 (0.211323) | 0.006048 / 0.007986 (-0.001937) | 0.005042 / 0.004328 (0.000714) | 0.102288 / 0.004250 (0.098038) | 0.048695 / 0.037052 (0.011643) | 0.559086 / 0.258489 (0.300597) | 0.553233 / 0.293841 (0.259392) | 0.044596 / 0.128546 (-0.083950) | 0.013696 / 0.075646 (-0.061950) | 0.109875 / 0.419271 (-0.309397) | 0.059993 / 0.043533 (0.016460) | 0.485579 / 0.255139 (0.230440) | 0.519835 / 0.283200 (0.236635) | 0.123504 / 0.141683 (-0.018179) | 1.820506 / 1.452155 (0.368351) | 1.963448 / 1.492716 (0.470732) |

### 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) | 0.292663 / 0.018006 (0.274656) | 0.557783 / 0.000490 (0.557293) | 0.001330 / 0.000200 (0.001130) | 0.000112 / 0.000054 (0.000057) |

### Benchmark: benchmark_indices_mapping.json

| metric | select | shard | shuffle | sort | train_test_split |
|--------|---|---|---|---|---|
| new / old (diff) | 0.036890 / 0.037411 (-0.000522) | 0.140373 / 0.014526 (0.125847) | 0.140176 / 0.176557 (-0.036381) | 0.237378 / 0.737135 (-0.499757) | 0.160186 / 0.296338 (-0.136152) |

### 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) | 0.673599 / 0.215209 (0.458390) | 6.510280 / 2.077655 (4.432625) | 2.981617 / 1.504120 (1.477497) | 2.684664 / 1.541195 (1.143469) | 2.760471 / 1.468490 (1.291981) | 0.975413 / 4.584777 (-3.609364) | 5.708933 / 3.745712 (1.963220) | 2.772069 / 5.269862 (-2.497793) | 1.763627 / 4.565676 (-2.802049) | 0.111632 / 0.424275 (-0.312643) | 0.013223 / 0.007607 (0.005616) | 0.791545 / 0.226044 (0.565500) | 8.063287 / 2.268929 (5.794359) | 3.671920 / 55.444624 (-51.772704) | 3.057248 / 6.876477 (-3.819229) | 3.083569 / 2.142072 (0.941497) | 1.118136 / 4.805227 (-3.687092) | 0.214655 / 6.500664 (-6.286009) | 0.083074 / 0.075469 (0.007605) |

### 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) | 1.761731 / 1.841788 (-0.080056) | 18.874200 / 8.074308 (10.799892) | 22.383693 / 10.191392 (12.192301) | 0.240292 / 0.680424 (-0.440132) | 0.028850 / 0.534201 (-0.505351) | 0.557334 / 0.579283 (-0.021949) | 0.627732 / 0.434364 (0.193369) | 0.634484 / 0.540337 (0.094146) | 0.767372 / 1.386936 (-0.619564) |

</details>
</details>

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