Support indexable objects in `Dataset.__getitem__`

As discussed in https://github.com/huggingface/datasets/pull/6816, this is needed to support objects that implement `__index__` such as `np.int64` in `Dataset.__getitem__`.

The docs for this PR live [here](https://moon-ci-docs.huggingface.co/docs/datasets/pr_6817). 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) | 0.005464 / 0.011353 (-0.005889) | 0.004174 / 0.011008 (-0.006834) | 0.064252 / 0.038508 (0.025744) | 0.033305 / 0.023109 (0.010196) | 0.245831 / 0.275898 (-0.030067) | 0.275575 / 0.323480 (-0.047905) | 0.003359 / 0.007986 (-0.004626) | 0.004196 / 0.004328 (-0.000132) | 0.049961 / 0.004250 (0.045710) | 0.048940 / 0.037052 (0.011888) | 0.261037 / 0.258489 (0.002548) | 0.295329 / 0.293841 (0.001488) | 0.028570 / 0.128546 (-0.099976) | 0.010747 / 0.075646 (-0.064900) | 0.216021 / 0.419271 (-0.203251) | 0.036885 / 0.043533 (-0.006648) | 0.251169 / 0.255139 (-0.003970) | 0.286233 / 0.283200 (0.003034) | 0.021253 / 0.141683 (-0.120429) | 1.150669 / 1.452155 (-0.301485) | 1.187577 / 1.492716 (-0.305140) |

### 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) | 0.094443 / 0.018006 (0.076436) | 0.304410 / 0.000490 (0.303920) | 0.000213 / 0.000200 (0.000013) | 0.000041 / 0.000054 (-0.000013) |

### Benchmark: benchmark_indices_mapping.json

| metric | select | shard | shuffle | sort | train_test_split |
|--------|---|---|---|---|---|
| new / old (diff) | 0.019568 / 0.037411 (-0.017844) | 0.065734 / 0.014526 (0.051208) | 0.076042 / 0.176557 (-0.100515) | 0.123624 / 0.737135 (-0.613511) | 0.078047 / 0.296338 (-0.218291) |

### 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) | 0.295725 / 0.215209 (0.080515) | 2.752501 / 2.077655 (0.674846) | 1.461856 / 1.504120 (-0.042264) | 1.353692 / 1.541195 (-0.187503) | 1.391777 / 1.468490 (-0.076713) | 0.563423 / 4.584777 (-4.021354) | 2.384620 / 3.745712 (-1.361092) | 2.876092 / 5.269862 (-2.393769) | 1.803913 / 4.565676 (-2.761763) | 0.062678 / 0.424275 (-0.361597) | 0.005428 / 0.007607 (-0.002179) | 0.333797 / 0.226044 (0.107753) | 3.304458 / 2.268929 (1.035530) | 1.801768 / 55.444624 (-53.642856) | 1.569406 / 6.876477 (-5.307070) | 1.614535 / 2.142072 (-0.527538) | 0.650178 / 4.805227 (-4.155049) | 0.119693 / 6.500664 (-6.380971) | 0.042832 / 0.075469 (-0.032637) |

### 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) | 0.982035 / 1.841788 (-0.859753) | 12.390006 / 8.074308 (4.315698) | 10.127018 / 10.191392 (-0.064374) | 0.131963 / 0.680424 (-0.548461) | 0.013926 / 0.534201 (-0.520275) | 0.289587 / 0.579283 (-0.289696) | 0.270302 / 0.434364 (-0.164062) | 0.327231 / 0.540337 (-0.213107) | 0.422522 / 1.386936 (-0.964414) |

</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) | 0.005666 / 0.011353 (-0.005687) | 0.003914 / 0.011008 (-0.007094) | 0.050315 / 0.038508 (0.011807) | 0.032367 / 0.023109 (0.009257) | 0.271732 / 0.275898 (-0.004166) | 0.297248 / 0.323480 (-0.026231) | 0.005101 / 0.007986 (-0.002884) | 0.002882 / 0.004328 (-0.001447) | 0.049651 / 0.004250 (0.045401) | 0.043773 / 0.037052 (0.006721) | 0.288011 / 0.258489 (0.029522) | 0.311863 / 0.293841 (0.018023) | 0.029147 / 0.128546 (-0.099399) | 0.010722 / 0.075646 (-0.064925) | 0.058832 / 0.419271 (-0.360440) | 0.033092 / 0.043533 (-0.010441) | 0.274686 / 0.255139 (0.019547) | 0.294174 / 0.283200 (0.010975) | 0.019196 / 0.141683 (-0.122486) | 1.126615 / 1.452155 (-0.325540) | 1.193107 / 1.492716 (-0.299609) |

### 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) | 0.097547 / 0.018006 (0.079541) | 0.316018 / 0.000490 (0.315529) | 0.000330 / 0.000200 (0.000130) | 0.000073 / 0.000054 (0.000019) |

### Benchmark: benchmark_indices_mapping.json

| metric | select | shard | shuffle | sort | train_test_split |
|--------|---|---|---|---|---|
| new / old (diff) | 0.022336 / 0.037411 (-0.015076) | 0.077092 / 0.014526 (0.062566) | 0.088873 / 0.176557 (-0.087684) | 0.128517 / 0.737135 (-0.608619) | 0.094061 / 0.296338 (-0.202278) |

### 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) | 0.300100 / 0.215209 (0.084891) | 2.893114 / 2.077655 (0.815460) | 1.570541 / 1.504120 (0.066421) | 1.453538 / 1.541195 (-0.087657) | 1.505325 / 1.468490 (0.036835) | 0.567955 / 4.584777 (-4.016822) | 2.458547 / 3.745712 (-1.287166) | 2.969181 / 5.269862 (-2.300680) | 1.850082 / 4.565676 (-2.715594) | 0.063811 / 0.424275 (-0.360464) | 0.005378 / 0.007607 (-0.002229) | 0.348219 / 0.226044 (0.122175) | 3.443986 / 2.268929 (1.175057) | 1.943005 / 55.444624 (-53.501620) | 1.686541 / 6.876477 (-5.189935) | 1.715552 / 2.142072 (-0.426520) | 0.641361 / 4.805227 (-4.163866) | 0.116652 / 6.500664 (-6.384012) | 0.042216 / 0.075469 (-0.033253) |

### 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) | 1.020102 / 1.841788 (-0.821686) | 12.966127 / 8.074308 (4.891819) | 10.748397 / 10.191392 (0.557005) | 0.132601 / 0.680424 (-0.547823) | 0.016643 / 0.534201 (-0.517558) | 0.289422 / 0.579283 (-0.289861) | 0.275524 / 0.434364 (-0.158840) | 0.332835 / 0.540337 (-0.207503) | 0.427867 / 1.386936 (-0.959069) |

</details>
</details>

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