Multi gpu docs

after discussions in https://github.com/huggingface/datasets/pull/6415


The docs for this PR live [here](https://moon-ci-docs.huggingface.co/docs/datasets/pr_6550). All of your documentation changes will be reflected on that endpoint. The docs are available until 30 days after the last update.

Thanks @lhoestq . This is a very important fix for code to run on multiple GPUs. Otherwise, only one GPU is working. I wish it can be merged soon. 
I also wrote a [blog post](https://forrestbao.github.io/2024/01/30/datasets_map_with_rank_multiple_GPUs.html) with a complete  example in case it can be helpful to someone. Please feel free to use complete example in any documentation. 


Thanks a lot @forrestbao ! I reused parts of your code for the documentation, I'm sure it will be useful to many people !

<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) | 0.005662 / 0.011353 (-0.005691) | 0.003930 / 0.011008 (-0.007078) | 0.063807 / 0.038508 (0.025299) | 0.030227 / 0.023109 (0.007118) | 0.235338 / 0.275898 (-0.040560) | 0.264433 / 0.323480 (-0.059047) | 0.004226 / 0.007986 (-0.003759) | 0.002847 / 0.004328 (-0.001481) | 0.048998 / 0.004250 (0.044747) | 0.042713 / 0.037052 (0.005660) | 0.250504 / 0.258489 (-0.007985) | 0.281101 / 0.293841 (-0.012740) | 0.029123 / 0.128546 (-0.099423) | 0.011388 / 0.075646 (-0.064258) | 0.211342 / 0.419271 (-0.207930) | 0.036437 / 0.043533 (-0.007096) | 0.238909 / 0.255139 (-0.016230) | 0.255853 / 0.283200 (-0.027347) | 0.018852 / 0.141683 (-0.122831) | 1.131870 / 1.452155 (-0.320284) | 1.209007 / 1.492716 (-0.283710) |

### 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) | 0.092433 / 0.018006 (0.074427) | 0.303045 / 0.000490 (0.302556) | 0.000291 / 0.000200 (0.000091) | 0.000079 / 0.000054 (0.000025) |

### Benchmark: benchmark_indices_mapping.json

| metric | select | shard | shuffle | sort | train_test_split |
|--------|---|---|---|---|---|
| new / old (diff) | 0.018349 / 0.037411 (-0.019062) | 0.062527 / 0.014526 (0.048002) | 0.075347 / 0.176557 (-0.101210) | 0.120587 / 0.737135 (-0.616549) | 0.075171 / 0.296338 (-0.221167) |

### 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) | 0.288364 / 0.215209 (0.073155) | 2.775779 / 2.077655 (0.698124) | 1.490875 / 1.504120 (-0.013245) | 1.375451 / 1.541195 (-0.165744) | 1.398923 / 1.468490 (-0.069567) | 0.588659 / 4.584777 (-3.996117) | 2.458114 / 3.745712 (-1.287598) | 2.928910 / 5.269862 (-2.340951) | 1.834221 / 4.565676 (-2.731456) | 0.064503 / 0.424275 (-0.359772) | 0.005028 / 0.007607 (-0.002580) | 0.340386 / 0.226044 (0.114341) | 3.408697 / 2.268929 (1.139769) | 1.843613 / 55.444624 (-53.601012) | 1.569300 / 6.876477 (-5.307177) | 1.636761 / 2.142072 (-0.505312) | 0.687854 / 4.805227 (-4.117374) | 0.123462 / 6.500664 (-6.377202) | 0.042877 / 0.075469 (-0.032593) |

### 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) | 0.984054 / 1.841788 (-0.857734) | 12.243934 / 8.074308 (4.169626) | 10.835244 / 10.191392 (0.643852) | 0.131609 / 0.680424 (-0.548815) | 0.014000 / 0.534201 (-0.520201) | 0.292070 / 0.579283 (-0.287213) | 0.271958 / 0.434364 (-0.162406) | 0.326866 / 0.540337 (-0.213471) | 0.440880 / 1.386936 (-0.946056) |

</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) | 0.005954 / 0.011353 (-0.005399) | 0.004123 / 0.011008 (-0.006885) | 0.050371 / 0.038508 (0.011863) | 0.034387 / 0.023109 (0.011277) | 0.273254 / 0.275898 (-0.002644) | 0.297785 / 0.323480 (-0.025695) | 0.004619 / 0.007986 (-0.003367) | 0.002884 / 0.004328 (-0.001444) | 0.050236 / 0.004250 (0.045986) | 0.048586 / 0.037052 (0.011533) | 0.283878 / 0.258489 (0.025389) | 0.315218 / 0.293841 (0.021377) | 0.060688 / 0.128546 (-0.067859) | 0.011991 / 0.075646 (-0.063655) | 0.059518 / 0.419271 (-0.359753) | 0.036113 / 0.043533 (-0.007420) | 0.274767 / 0.255139 (0.019628) | 0.290620 / 0.283200 (0.007420) | 0.020070 / 0.141683 (-0.121613) | 1.164635 / 1.452155 (-0.287519) | 1.189482 / 1.492716 (-0.303234) |

### 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) | 0.095171 / 0.018006 (0.077165) | 0.307129 / 0.000490 (0.306639) | 0.000227 / 0.000200 (0.000027) | 0.000045 / 0.000054 (-0.000009) |

### Benchmark: benchmark_indices_mapping.json

| metric | select | shard | shuffle | sort | train_test_split |
|--------|---|---|---|---|---|
| new / old (diff) | 0.022777 / 0.037411 (-0.014634) | 0.076761 / 0.014526 (0.062235) | 0.087654 / 0.176557 (-0.088902) | 0.126729 / 0.737135 (-0.610406) | 0.089491 / 0.296338 (-0.206847) |

### 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) | 0.292208 / 0.215209 (0.076999) | 2.890491 / 2.077655 (0.812836) | 1.625696 / 1.504120 (0.121576) | 1.463484 / 1.541195 (-0.077710) | 1.490889 / 1.468490 (0.022399) | 0.582155 / 4.584777 (-4.002622) | 2.492209 / 3.745712 (-1.253503) | 2.817020 / 5.269862 (-2.452842) | 1.806812 / 4.565676 (-2.758864) | 0.065830 / 0.424275 (-0.358445) | 0.005089 / 0.007607 (-0.002518) | 0.356067 / 0.226044 (0.130022) | 3.489652 / 2.268929 (1.220723) | 1.959276 / 55.444624 (-53.485348) | 1.678819 / 6.876477 (-5.197657) | 1.853581 / 2.142072 (-0.288491) | 0.660515 / 4.805227 (-4.144712) | 0.119884 / 6.500664 (-6.380780) | 0.041713 / 0.075469 (-0.033757) |

### 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) | 1.021701 / 1.841788 (-0.820087) | 12.918290 / 8.074308 (4.843982) | 11.469371 / 10.191392 (1.277979) | 0.144830 / 0.680424 (-0.535594) | 0.015858 / 0.534201 (-0.518343) | 0.290136 / 0.579283 (-0.289148) | 0.277894 / 0.434364 (-0.156470) | 0.330091 / 0.540337 (-0.210247) | 0.422697 / 1.386936 (-0.964240) |

</details>
</details>

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