AssertionError when using label_cols in to_tf_dataset 
## Describe the bug
An incorrect `AssertionError` is raised when using `label_cols` in `to_tf_dataset` and the label's key name is `label`.

The assertion is in this line:
https://github.com/huggingface/datasets/blob/2.4.0/src/datasets/arrow_dataset.py#L475

## Steps to reproduce the bug
```python
from datasets import load_dataset
from transformers import DefaultDataCollator

dataset = load_dataset('glue', 'mrpc', split='train')

tf_dataset = dataset.to_tf_dataset(
    columns=["sentence1", "sentence2", "idx"],
    label_cols=["label"],
    batch_size=16,
    collate_fn=DefaultDataCollator(return_tensors="tf"),
)
```

## Expected results
No assertion error.

## Actual results
```
AssertionError: in user code:

    File "/opt/conda/lib/python3.8/site-packages/datasets/arrow_dataset.py", line 475, in split_features_and_labels  *
        assert set(features.keys()).union(labels.keys()) == set(input_batch.keys())
```

## Environment info
<!-- You can run the command `datasets-cli env` and copy-and-paste its output below. -->
- `datasets` version: 2.4.0
- Platform: Linux-4.18.0-305.45.1.el8_4.ppc64le-ppc64le-with-glibc2.17
- Python version: 3.8.13
- PyArrow version: 7.0.0
- Pandas version: 1.4.3
