Fallback on dataset script if user wants to load default config

Right now this code is failing on `main`:

```python
load_dataset("openbookqa")
```

This is because it tries to load the dataset from the Parquet export but the dataset has multiple configurations and the Parquet export doesn't know which one is the default one.

I fixed this by simply falling back on using the dataset script (which tells the user to pass `trust_remote_code=True`):

```python
load_dataset("openbookqa", trust_remote_code=True)
```

Note that if the user happened to specify a config name I don't fall back on the script since we can use the Parquet export in this case (no need to know which config is the default)


```python
load_dataset("openbookqa", "main")
```

The docs for this PR live [here](https://moon-ci-docs.huggingface.co/docs/datasets/pr_6498). All of your documentation changes will be reflected on that endpoint. The docs are available until 30 days after the last update.

> I was just thinking: what if the user does not pass a config name and the dataset has only a config with a name different from "default"?

You mean if there is a DEFAULT_CONFIG_NAME defined in the script but the dataset only has one configuration ? We can't easily get the number of configs without running the python code so I don't think we can support detect this case


Most datasets with a script don't define DEFAULT_CONFIG_NAME if there is only one configuration anyway.

So there is no issue e.g. for `squad`

> I was trying to mean the case where DEFAULT_CONFIG_NAME is None but there is only a single config in BUILDER_CONFIGS, with a name different from "default".

In this case we can detect if "DEFAULT_CONFIG_NAME" is not mentioned and use the Parquet export. If it is mentioned (and maybe it is set to None or to the single config) I consider that it may have multiple configs and fall back on using the script

... but the user does not pass the config name.

In this case we load the single configuration (this is how a DatasetBuilder works)

see 

https://github.com/huggingface/datasets/blob/2feaa589de86dd85941301fc8c3fa091731a67c0/src/datasets/builder.py#L532-L532

<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) | 0.005122 / 0.011353 (-0.006231) | 0.003565 / 0.011008 (-0.007443) | 0.062706 / 0.038508 (0.024198) | 0.049314 / 0.023109 (0.026205) | 0.247325 / 0.275898 (-0.028573) | 0.269788 / 0.323480 (-0.053692) | 0.003895 / 0.007986 (-0.004090) | 0.002788 / 0.004328 (-0.001540) | 0.048615 / 0.004250 (0.044365) | 0.037591 / 0.037052 (0.000539) | 0.253495 / 0.258489 (-0.004994) | 0.281200 / 0.293841 (-0.012641) | 0.027712 / 0.128546 (-0.100834) | 0.010901 / 0.075646 (-0.064745) | 0.205577 / 0.419271 (-0.213694) | 0.035989 / 0.043533 (-0.007544) | 0.252978 / 0.255139 (-0.002161) | 0.268042 / 0.283200 (-0.015157) | 0.017857 / 0.141683 (-0.123826) | 1.096633 / 1.452155 (-0.355521) | 1.147026 / 1.492716 (-0.345691) |

### 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) | 0.095609 / 0.018006 (0.077603) | 0.311941 / 0.000490 (0.311451) | 0.000211 / 0.000200 (0.000011) | 0.000043 / 0.000054 (-0.000012) |

### Benchmark: benchmark_indices_mapping.json

| metric | select | shard | shuffle | sort | train_test_split |
|--------|---|---|---|---|---|
| new / old (diff) | 0.019042 / 0.037411 (-0.018369) | 0.060549 / 0.014526 (0.046023) | 0.074761 / 0.176557 (-0.101796) | 0.121729 / 0.737135 (-0.615406) | 0.075661 / 0.296338 (-0.220677) |

### 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) | 0.284774 / 0.215209 (0.069565) | 2.764576 / 2.077655 (0.686921) | 1.489926 / 1.504120 (-0.014194) | 1.387276 / 1.541195 (-0.153919) | 1.400931 / 1.468490 (-0.067559) | 0.555623 / 4.584777 (-4.029154) | 2.409488 / 3.745712 (-1.336224) | 2.781053 / 5.269862 (-2.488808) | 1.750472 / 4.565676 (-2.815204) | 0.062232 / 0.424275 (-0.362043) | 0.004974 / 0.007607 (-0.002633) | 0.336324 / 0.226044 (0.110280) | 3.286619 / 2.268929 (1.017691) | 1.825070 / 55.444624 (-53.619554) | 1.537993 / 6.876477 (-5.338484) | 1.586520 / 2.142072 (-0.555553) | 0.640090 / 4.805227 (-4.165138) | 0.117637 / 6.500664 (-6.383027) | 0.042318 / 0.075469 (-0.033151) |

### 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) | 0.964051 / 1.841788 (-0.877736) | 11.706259 / 8.074308 (3.631951) | 10.752311 / 10.191392 (0.560919) | 0.128117 / 0.680424 (-0.552307) | 0.014001 / 0.534201 (-0.520200) | 0.286255 / 0.579283 (-0.293028) | 0.263810 / 0.434364 (-0.170554) | 0.329347 / 0.540337 (-0.210991) | 0.437349 / 1.386936 (-0.949587) |

</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) | 0.005303 / 0.011353 (-0.006050) | 0.003586 / 0.011008 (-0.007422) | 0.049339 / 0.038508 (0.010831) | 0.051287 / 0.023109 (0.028178) | 0.274397 / 0.275898 (-0.001501) | 0.292977 / 0.323480 (-0.030503) | 0.004029 / 0.007986 (-0.003957) | 0.002727 / 0.004328 (-0.001602) | 0.048779 / 0.004250 (0.044528) | 0.040075 / 0.037052 (0.003022) | 0.277676 / 0.258489 (0.019187) | 0.301963 / 0.293841 (0.008122) | 0.029340 / 0.128546 (-0.099206) | 0.010714 / 0.075646 (-0.064932) | 0.057253 / 0.419271 (-0.362018) | 0.033426 / 0.043533 (-0.010107) | 0.276673 / 0.255139 (0.021534) | 0.291053 / 0.283200 (0.007854) | 0.017660 / 0.141683 (-0.124023) | 1.122354 / 1.452155 (-0.329800) | 1.180381 / 1.492716 (-0.312335) |

### 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) | 0.091903 / 0.018006 (0.073897) | 0.300720 / 0.000490 (0.300231) | 0.000288 / 0.000200 (0.000088) | 0.000044 / 0.000054 (-0.000011) |

### Benchmark: benchmark_indices_mapping.json

| metric | select | shard | shuffle | sort | train_test_split |
|--------|---|---|---|---|---|
| new / old (diff) | 0.021521 / 0.037411 (-0.015890) | 0.068233 / 0.014526 (0.053707) | 0.081245 / 0.176557 (-0.095312) | 0.119996 / 0.737135 (-0.617139) | 0.082483 / 0.296338 (-0.213856) |

### 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) | 0.302776 / 0.215209 (0.087567) | 2.950776 / 2.077655 (0.873122) | 1.631032 / 1.504120 (0.126912) | 1.502021 / 1.541195 (-0.039174) | 1.514213 / 1.468490 (0.045723) | 0.578246 / 4.584777 (-4.006531) | 2.443768 / 3.745712 (-1.301944) | 2.827811 / 5.269862 (-2.442051) | 1.771529 / 4.565676 (-2.794148) | 0.064479 / 0.424275 (-0.359797) | 0.005061 / 0.007607 (-0.002546) | 0.350966 / 0.226044 (0.124922) | 3.458616 / 2.268929 (1.189687) | 1.967917 / 55.444624 (-53.476707) | 1.704661 / 6.876477 (-5.171815) | 1.698895 / 2.142072 (-0.443178) | 0.663259 / 4.805227 (-4.141968) | 0.122140 / 6.500664 (-6.378525) | 0.041099 / 0.075469 (-0.034371) |

### 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) | 0.972080 / 1.841788 (-0.869708) | 12.123286 / 8.074308 (4.048978) | 10.819854 / 10.191392 (0.628462) | 0.131486 / 0.680424 (-0.548938) | 0.015785 / 0.534201 (-0.518416) | 0.290048 / 0.579283 (-0.289235) | 0.277822 / 0.434364 (-0.156542) | 0.325949 / 0.540337 (-0.214388) | 0.577681 / 1.386936 (-0.809255) |

</details>
</details>

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