Add num_test_batches option

`to_tf_dataset` calls can be very costly because of the number of test batches drawn during `_get_output_signature`. The test batches are draw in order to estimate the shapes when creating the tensorflow dataset. This is necessary when the shapes can be irregular, but not in cases when the tensor shapes are the same across all samples. This PR adds an option to change the number of batches drawn, so the user can speed this conversion up. 

Running the following, and modifying `num_test_batches`
```
import time
from datasets import load_dataset
from transformers import DefaultDataCollator

data_collator = DefaultDataCollator()
dataset = load_dataset("beans")
dataset = dataset["train"].with_format("np")
start = time.time()
dataset = dataset.to_tf_dataset(
    columns=["image"],
    label_cols=["label"],
    batch_size=8,
    collate_fn=data_collator,
    num_test_batches=NUM_TEST_BATCHES,
)
end = time.time()
print(end - start)
```

NUM_TEST_BATCHES=200:  0.8197s
NUM_TEST_BATCHES=50:    0.3070s
NUM_TEST_BATCHES=2:      0.1417s
NUM_TEST_BATCHES=1:      0.1352s 

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

I thought this issue was resolved in my parallel `to_tf_dataset` PR! I changed the default `num_test_batches` in `_get_output_signature` to 20 and used a test batch size of 1 to maximize variance to detect shorter samples. I think it's still okay to have this PR, though - but I'd use the new value of 20 as the default!

@Rocketknight1  You're right - I didn't have the most recent changes to the default values. Updated now to 20! I still think it would be good to have it configurable from the `to_tf_dataset` call so the user has the option to either make it more robust if many samples are needed, or faster if only one is needed.  That, and I selfishly want it for faster tests. 

<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) | 0.010441 / 0.011353 (-0.000912) | 0.005605 / 0.011008 (-0.005404) | 0.115712 / 0.038508 (0.077204) | 0.040907 / 0.023109 (0.017797) | 0.357673 / 0.275898 (0.081775) | 0.415427 / 0.323480 (0.091947) | 0.008827 / 0.007986 (0.000842) | 0.006069 / 0.004328 (0.001740) | 0.088985 / 0.004250 (0.084735) | 0.048461 / 0.037052 (0.011409) | 0.362065 / 0.258489 (0.103576) | 0.393643 / 0.293841 (0.099802) | 0.043844 / 0.128546 (-0.084703) | 0.013757 / 0.075646 (-0.061889) | 0.390993 / 0.419271 (-0.028278) | 0.053612 / 0.043533 (0.010079) | 0.348688 / 0.255139 (0.093549) | 0.377818 / 0.283200 (0.094619) | 0.115762 / 0.141683 (-0.025920) | 1.751826 / 1.452155 (0.299672) | 1.773326 / 1.492716 (0.280609) |

### 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) | 0.220668 / 0.018006 (0.202662) | 0.536830 / 0.000490 (0.536340) | 0.000467 / 0.000200 (0.000267) | 0.000069 / 0.000054 (0.000015) |

### Benchmark: benchmark_indices_mapping.json

| metric | select | shard | shuffle | sort | train_test_split |
|--------|---|---|---|---|---|
| new / old (diff) | 0.031500 / 0.037411 (-0.005911) | 0.125796 / 0.014526 (0.111270) | 0.137539 / 0.176557 (-0.039017) | 0.184651 / 0.737135 (-0.552484) | 0.145707 / 0.296338 (-0.150632) |

### 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) | 0.465876 / 0.215209 (0.250667) | 4.637711 / 2.077655 (2.560056) | 2.132335 / 1.504120 (0.628215) | 1.862593 / 1.541195 (0.321398) | 1.961701 / 1.468490 (0.493211) | 0.800551 / 4.584777 (-3.784226) | 4.453321 / 3.745712 (0.707608) | 4.291030 / 5.269862 (-0.978832) | 2.256685 / 4.565676 (-2.308991) | 0.097787 / 0.424275 (-0.326488) | 0.014116 / 0.007607 (0.006509) | 0.593395 / 0.226044 (0.367351) | 5.885774 / 2.268929 (3.616845) | 2.666224 / 55.444624 (-52.778400) | 2.276673 / 6.876477 (-4.599803) | 2.358190 / 2.142072 (0.216117) | 0.981398 / 4.805227 (-3.823829) | 0.196997 / 6.500664 (-6.303668) | 0.077020 / 0.075469 (0.001550) |

### 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) | 1.365646 / 1.841788 (-0.476142) | 17.418157 / 8.074308 (9.343849) | 15.838749 / 10.191392 (5.647357) | 0.172749 / 0.680424 (-0.507675) | 0.033711 / 0.534201 (-0.500490) | 0.513306 / 0.579283 (-0.065978) | 0.503201 / 0.434364 (0.068837) | 0.608954 / 0.540337 (0.068616) | 0.734697 / 1.386936 (-0.652239) |

</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) | 0.008749 / 0.011353 (-0.002604) | 0.005738 / 0.011008 (-0.005270) | 0.084946 / 0.038508 (0.046438) | 0.040386 / 0.023109 (0.017277) | 0.398698 / 0.275898 (0.122800) | 0.435843 / 0.323480 (0.112363) | 0.006812 / 0.007986 (-0.001174) | 0.004567 / 0.004328 (0.000239) | 0.085857 / 0.004250 (0.081607) | 0.054791 / 0.037052 (0.017738) | 0.400381 / 0.258489 (0.141892) | 0.460313 / 0.293841 (0.166472) | 0.042299 / 0.128546 (-0.086247) | 0.014128 / 0.075646 (-0.061519) | 0.100497 / 0.419271 (-0.318775) | 0.058356 / 0.043533 (0.014823) | 0.399774 / 0.255139 (0.144635) | 0.428210 / 0.283200 (0.145011) | 0.122084 / 0.141683 (-0.019598) | 1.683519 / 1.452155 (0.231365) | 1.798024 / 1.492716 (0.305307) |

### 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) | 0.255058 / 0.018006 (0.237051) | 0.488831 / 0.000490 (0.488342) | 0.008349 / 0.000200 (0.008149) | 0.000183 / 0.000054 (0.000129) |

### Benchmark: benchmark_indices_mapping.json

| metric | select | shard | shuffle | sort | train_test_split |
|--------|---|---|---|---|---|
| new / old (diff) | 0.034870 / 0.037411 (-0.002541) | 0.131818 / 0.014526 (0.117292) | 0.143607 / 0.176557 (-0.032949) | 0.197413 / 0.737135 (-0.539722) | 0.148970 / 0.296338 (-0.147368) |

### 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) | 0.492831 / 0.215209 (0.277622) | 4.963085 / 2.077655 (2.885430) | 2.367803 / 1.504120 (0.863683) | 2.145535 / 1.541195 (0.604340) | 2.289452 / 1.468490 (0.820962) | 0.812691 / 4.584777 (-3.772086) | 4.554068 / 3.745712 (0.808356) | 2.377126 / 5.269862 (-2.892735) | 1.537243 / 4.565676 (-3.028433) | 0.099742 / 0.424275 (-0.324534) | 0.014757 / 0.007607 (0.007149) | 0.628714 / 0.226044 (0.402670) | 6.240197 / 2.268929 (3.971268) | 2.961929 / 55.444624 (-52.482696) | 2.533436 / 6.876477 (-4.343040) | 2.642619 / 2.142072 (0.500547) | 0.976002 / 4.805227 (-3.829225) | 0.197912 / 6.500664 (-6.302752) | 0.078767 / 0.075469 (0.003297) |

### 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) | 1.522863 / 1.841788 (-0.318925) | 18.210504 / 8.074308 (10.136196) | 15.664172 / 10.191392 (5.472780) | 0.178510 / 0.680424 (-0.501914) | 0.020852 / 0.534201 (-0.513349) | 0.501757 / 0.579283 (-0.077526) | 0.496542 / 0.434364 (0.062178) | 0.624958 / 0.540337 (0.084620) | 0.746960 / 1.386936 (-0.639976) |

</details>
</details>

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