Title: Add support for categorical/dictionary types
State: closed

Arrow has a very useful dictionary/categorical type (https://arrow.apache.org/docs/python/generated/pyarrow.dictionary.html). This data type has significant speed, memory and disk benefits over pa.string() when there are only a few unique text strings in a column.

Unfortunately, huggingface datasets currently does not support this type. So huggingface datasets cannot natively read many parquet files that use this datatype .This PR adds support for Huggingface Datasets to read categorical/dictionary data.

Note: This PR functions by simply converting those dictionary/categorical types to strings. This means that huggingface datasets cannot take advantage of the compute benefits of categoricals, but it significantly simplifies logic. At this time, I do not think it makes sense to optimize categorical support within huggingface datasets and that we should only try to optimize later, if necessary.

Closes #5706

Comments: The docs for this PR live [here](https://moon-ci-docs.huggingface.co/docs/datasets/pr_6892). All of your documentation changes will be reflected on that endpoint. The docs are available until 30 days after the last update.<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) | 0.005388 / 0.011353 (-0.005965) | 0.004004 / 0.011008 (-0.007005) | 0.064037 / 0.038508 (0.025529) | 0.031666 / 0.023109 (0.008557) | 0.236493 / 0.275898 (-0.039405) | 0.269047 / 0.323480 (-0.054432) | 0.005008 / 0.007986 (-0.002977) | 0.002964 / 0.004328 (-0.001364) | 0.049926 / 0.004250 (0.045675) | 0.048092 / 0.037052 (0.011039) | 0.245563 / 0.258489 (-0.012926) | 0.282614 / 0.293841 (-0.011227) | 0.027488 / 0.128546 (-0.101058) | 0.010904 / 0.075646 (-0.064742) | 0.204892 / 0.419271 (-0.214379) | 0.037161 / 0.043533 (-0.006372) | 0.238488 / 0.255139 (-0.016651) | 0.258192 / 0.283200 (-0.025008) | 0.018819 / 0.141683 (-0.122864) | 1.131573 / 1.452155 (-0.320582) | 1.204084 / 1.492716 (-0.288632) |

### 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) | 0.095852 / 0.018006 (0.077846) | 0.300225 / 0.000490 (0.299735) | 0.000217 / 0.000200 (0.000017) | 0.000052 / 0.000054 (-0.000002) |

### Benchmark: benchmark_indices_mapping.json

| metric | select | shard | shuffle | sort | train_test_split |
|--------|---|---|---|---|---|
| new / old (diff) | 0.018592 / 0.037411 (-0.018819) | 0.062297 / 0.014526 (0.047772) | 0.074344 / 0.176557 (-0.102212) | 0.120654 / 0.737135 (-0.616481) | 0.075567 / 0.296338 (-0.220772) |

### 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) | 0.287700 / 0.215209 (0.072491) | 2.829536 / 2.077655 (0.751882) | 1.446296 / 1.504120 (-0.057824) | 1.320912 / 1.541195 (-0.220283) | 1.362744 / 1.468490 (-0.105746) | 0.563732 / 4.584777 (-4.021045) | 2.399904 / 3.745712 (-1.345808) | 2.676706 / 5.269862 (-2.593156) | 1.744780 / 4.565676 (-2.820896) | 0.062884 / 0.424275 (-0.361391) | 0.004936 / 0.007607 (-0.002671) | 0.338084 / 0.226044 (0.112040) | 3.309532 / 2.268929 (1.040603) | 1.792791 / 55.444624 (-53.651833) | 1.502038 / 6.876477 (-5.374439) | 1.662417 / 2.142072 (-0.479655) | 0.642835 / 4.805227 (-4.162393) | 0.117002 / 6.500664 (-6.383662) | 0.041880 / 0.075469 (-0.033589) |

### 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) | 0.974814 / 1.841788 (-0.866974) | 11.430883 / 8.074308 (3.356575) | 10.314734 / 10.191392 (0.123342) | 0.139838 / 0.680424 (-0.540586) | 0.014939 / 0.534201 (-0.519262) | 0.288048 / 0.579283 (-0.291235) | 0.269146 / 0.434364 (-0.165218) | 0.324300 / 0.540337 (-0.216037) | 0.421612 / 1.386936 (-0.965324) |

</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) | 0.005660 / 0.011353 (-0.005692) | 0.003723 / 0.011008 (-0.007285) | 0.049909 / 0.038508 (0.011401) | 0.033079 / 0.023109 (0.009970) | 0.270940 / 0.275898 (-0.004958) | 0.291173 / 0.323480 (-0.032307) | 0.004336 / 0.007986 (-0.003650) | 0.002793 / 0.004328 (-0.001535) | 0.049619 / 0.004250 (0.045368) | 0.041062 / 0.037052 (0.004010) | 0.285026 / 0.258489 (0.026537) | 0.322119 / 0.293841 (0.028278) | 0.029653 / 0.128546 (-0.098894) | 0.010785 / 0.075646 (-0.064861) | 0.058680 / 0.419271 (-0.360591) | 0.033300 / 0.043533 (-0.010233) | 0.269452 / 0.255139 (0.014313) | 0.285426 / 0.283200 (0.002226) | 0.017655 / 0.141683 (-0.124028) | 1.144713 / 1.452155 (-0.307442) | 1.196828 / 1.492716 (-0.295888) |

### 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) | 0.096719 / 0.018006 (0.078713) | 0.303532 / 0.000490 (0.303042) | 0.000223 / 0.000200 (0.000023) | 0.000049 / 0.000054 (-0.000005) |

### Benchmark: benchmark_indices_mapping.json

| metric | select | shard | shuffle | sort | train_test_split |
|--------|---|---|---|---|---|
| new / old (diff) | 0.022620 / 0.037411 (-0.014791) | 0.077057 / 0.014526 (0.062532) | 0.088570 / 0.176557 (-0.087987) | 0.128715 / 0.737135 (-0.608421) | 0.090844 / 0.296338 (-0.205494) |

### 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) | 0.298101 / 0.215209 (0.082892) | 2.919861 / 2.077655 (0.842206) | 1.608945 / 1.504120 (0.104825) | 1.487756 / 1.541195 (-0.053439) | 1.520800 / 1.468490 (0.052310) | 0.576615 / 4.584777 (-4.008162) | 0.964250 / 3.745712 (-2.781462) | 2.852968 / 5.269862 (-2.416893) | 1.868768 / 4.565676 (-2.696908) | 0.063934 / 0.424275 (-0.360341) | 0.005093 / 0.007607 (-0.002514) | 0.352984 / 0.226044 (0.126939) | 3.507441 / 2.268929 (1.238513) | 1.944467 / 55.444624 (-53.500158) | 1.663985 / 6.876477 (-5.212492) | 1.847029 / 2.142072 (-0.295043) | 0.669228 / 4.805227 (-4.136000) | 0.118990 / 6.500664 (-6.381675) | 0.041788 / 0.075469 (-0.033681) |

### 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) | 1.004541 / 1.841788 (-0.837247) | 12.525181 / 8.074308 (4.450873) | 10.488167 / 10.191392 (0.296775) | 0.141182 / 0.680424 (-0.539242) | 0.016432 / 0.534201 (-0.517769) | 0.283682 / 0.579283 (-0.295601) | 0.128277 / 0.434364 (-0.306087) | 0.321933 / 0.540337 (-0.218404) | 0.416430 / 1.386936 (-0.970506) |

</details>
</details>

![](https://cml.dev/watermark.png#686f5df47442bf4b3a2a73ba255427ae8d659eea "CML watermark")
@lhoestq  Thanks a ton for helping this get merged!