Support streaming datasets with os.path.exists and Path.exists

Support streaming datasets with `os.path.exists` and `pathlib.Path.exists`.

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

<details>
<summary>Show benchmarks</summary>

PyArrow==6.0.0

<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) | 0.008638 / 0.011353 (-0.002715) | 0.004565 / 0.011008 (-0.006444) | 0.098984 / 0.038508 (0.060476) | 0.030118 / 0.023109 (0.007009) | 0.321779 / 0.275898 (0.045881) | 0.366905 / 0.323480 (0.043426) | 0.006931 / 0.007986 (-0.001055) | 0.004728 / 0.004328 (0.000399) | 0.078358 / 0.004250 (0.074108) | 0.037755 / 0.037052 (0.000702) | 0.312694 / 0.258489 (0.054205) | 0.351781 / 0.293841 (0.057940) | 0.033266 / 0.128546 (-0.095280) | 0.011397 / 0.075646 (-0.064250) | 0.323501 / 0.419271 (-0.095771) | 0.040779 / 0.043533 (-0.002754) | 0.303533 / 0.255139 (0.048394) | 0.340940 / 0.283200 (0.057740) | 0.088701 / 0.141683 (-0.052982) | 1.472058 / 1.452155 (0.019904) | 1.529535 / 1.492716 (0.036818) |

### 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) | 0.191803 / 0.018006 (0.173797) | 0.409773 / 0.000490 (0.409283) | 0.002704 / 0.000200 (0.002504) | 0.000217 / 0.000054 (0.000163) |

### Benchmark: benchmark_indices_mapping.json

| metric | select | shard | shuffle | sort | train_test_split |
|--------|---|---|---|---|---|
| new / old (diff) | 0.023520 / 0.037411 (-0.013891) | 0.096967 / 0.014526 (0.082441) | 0.107911 / 0.176557 (-0.068646) | 0.146425 / 0.737135 (-0.590710) | 0.109025 / 0.296338 (-0.187314) |

### 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) | 0.418565 / 0.215209 (0.203356) | 4.183429 / 2.077655 (2.105774) | 1.886534 / 1.504120 (0.382414) | 1.689015 / 1.541195 (0.147820) | 1.710757 / 1.468490 (0.242267) | 0.693211 / 4.584777 (-3.891566) | 3.380062 / 3.745712 (-0.365650) | 2.619910 / 5.269862 (-2.649952) | 1.457512 / 4.565676 (-3.108164) | 0.082421 / 0.424275 (-0.341854) | 0.012126 / 0.007607 (0.004519) | 0.525249 / 0.226044 (0.299205) | 5.244541 / 2.268929 (2.975613) | 2.305908 / 55.444624 (-53.138717) | 1.945298 / 6.876477 (-4.931178) | 2.015618 / 2.142072 (-0.126455) | 0.816746 / 4.805227 (-3.988481) | 0.148325 / 6.500664 (-6.352339) | 0.063939 / 0.075469 (-0.011530) |

### 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) | 1.255790 / 1.841788 (-0.585998) | 13.433219 / 8.074308 (5.358911) | 13.916957 / 10.191392 (3.725565) | 0.153468 / 0.680424 (-0.526956) | 0.028722 / 0.534201 (-0.505479) | 0.398245 / 0.579283 (-0.181038) | 0.399067 / 0.434364 (-0.035296) | 0.457525 / 0.540337 (-0.082812) | 0.542391 / 1.386936 (-0.844545) |

</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) | 0.006411 / 0.011353 (-0.004942) | 0.004552 / 0.011008 (-0.006456) | 0.098036 / 0.038508 (0.059527) | 0.026532 / 0.023109 (0.003422) | 0.412270 / 0.275898 (0.136372) | 0.442771 / 0.323480 (0.119291) | 0.004891 / 0.007986 (-0.003094) | 0.003488 / 0.004328 (-0.000841) | 0.075437 / 0.004250 (0.071186) | 0.036228 / 0.037052 (-0.000824) | 0.413246 / 0.258489 (0.154757) | 0.453546 / 0.293841 (0.159705) | 0.031054 / 0.128546 (-0.097492) | 0.011589 / 0.075646 (-0.064058) | 0.318477 / 0.419271 (-0.100794) | 0.041075 / 0.043533 (-0.002457) | 0.411182 / 0.255139 (0.156043) | 0.436991 / 0.283200 (0.153792) | 0.086563 / 0.141683 (-0.055120) | 1.511948 / 1.452155 (0.059793) | 1.570925 / 1.492716 (0.078208) |

### 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) | 0.200510 / 0.018006 (0.182504) | 0.403450 / 0.000490 (0.402960) | 0.000397 / 0.000200 (0.000197) | 0.000058 / 0.000054 (0.000003) |

### Benchmark: benchmark_indices_mapping.json

| metric | select | shard | shuffle | sort | train_test_split |
|--------|---|---|---|---|---|
| new / old (diff) | 0.023950 / 0.037411 (-0.013461) | 0.097334 / 0.014526 (0.082808) | 0.105228 / 0.176557 (-0.071328) | 0.137699 / 0.737135 (-0.599436) | 0.107063 / 0.296338 (-0.189275) |

### 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) | 0.474420 / 0.215209 (0.259211) | 4.748212 / 2.077655 (2.670557) | 2.407318 / 1.504120 (0.903198) | 2.198949 / 1.541195 (0.657755) | 2.220377 / 1.468490 (0.751887) | 0.704022 / 4.584777 (-3.880755) | 3.366128 / 3.745712 (-0.379584) | 1.839454 / 5.269862 (-3.430408) | 1.151183 / 4.565676 (-3.414493) | 0.082818 / 0.424275 (-0.341457) | 0.012765 / 0.007607 (0.005158) | 0.571913 / 0.226044 (0.345868) | 5.722544 / 2.268929 (3.453615) | 2.858279 / 55.444624 (-52.586346) | 2.513479 / 6.876477 (-4.362998) | 2.574227 / 2.142072 (0.432154) | 0.803282 / 4.805227 (-4.001945) | 0.150603 / 6.500664 (-6.350061) | 0.066594 / 0.075469 (-0.008875) |

### 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) | 1.301161 / 1.841788 (-0.540627) | 13.580745 / 8.074308 (5.506436) | 13.301551 / 10.191392 (3.110159) | 0.141424 / 0.680424 (-0.539000) | 0.016579 / 0.534201 (-0.517622) | 0.380726 / 0.579283 (-0.198557) | 0.383011 / 0.434364 (-0.051353) | 0.438717 / 0.540337 (-0.101620) | 0.527085 / 1.386936 (-0.859851) |

</details>
</details>

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