For data-only datasets, streaming and non-streaming don't behave the same
We should at least raise an error if a new sample has column names that are missing, or if it has extra columns. No need to check for the type for now.

I'm in favor of having an error especially because we want to avoid silent issues as much as possible - i.e. when something goes wrong (when schemas don't match or some data are missing) and no errors/warnings are raised.

Consistency between streaming and non-streaming is also important.
See https://huggingface.co/datasets/huggingface/transformers-metadata: it only contains two JSON files.

In streaming mode, the files are concatenated, and thus the rows might be dictionaries with different keys:

```python
import datasets as ds
iterable_dataset = ds.load_dataset("huggingface/transformers-metadata", split="train", streaming=True);
rows = list(iterable_dataset.take(100))
rows[0]
# {'model_type': 'albert', 'pytorch': True, 'tensorflow': True, 'flax': True, 'processor': 'AutoTokenizer'}
rows[99]
# {'model_class': 'BartModel', 'pipeline_tag': 'feature-extraction', 'auto_class': 'AutoModel'}
```

In normal mode, an exception is thrown:

```python
import datasets as ds
dataset = ds.load_dataset("huggingface/transformers-metadata", split="train");
```

```
ValueError: Couldn't cast
model_class: string
pipeline_tag: string
auto_class: string
to
{'model_type': Value(dtype='string', id=None), 'pytorch': Value(dtype='bool', id=None), 'tensorflow': Value(dtype='bool', id=None), 'flax': Value(dtype='bool', id=None), 'processor': Value(dtype='string', id=None)}
because column names don't match
```
For data-only datasets, streaming and non-streaming don't behave the same 
 See https://huggingface.co/datasets/huggingface/transformers-metadata: it only contains two JSON files.

In streaming mode, the files are concatenated, and thus the rows might be dictionaries with different keys:

```python
import datasets as ds
iterable_dataset = ds.load_dataset("huggingface/transformers-metadata", split="train", streaming=True);
rows = list(iterable_dataset.take(100))
rows[0]
# {'model_type': 'albert', 'pytorch': True, 'tensorflow': True, 'flax': True, 'processor': 'AutoTokenizer'}
rows[99]
# {'model_class': 'BartModel', 'pipeline_tag': 'feature-extraction', 'auto_class': 'AutoModel'}
```

In normal mode, an exception is thrown:

```python
import datasets as ds
dataset = ds.load_dataset("huggingface/transformers-metadata", split="train");
```

```
ValueError: Couldn't cast
model_class: string
pipeline_tag: string
auto_class: string
to
{'model_type': Value(dtype='string', id=None), 'pytorch': Value(dtype='bool', id=None), 'tensorflow': Value(dtype='bool', id=None), 'flax': Value(dtype='bool', id=None), 'processor': Value(dtype='string', id=None)}
because column names don't match
``` 
 We should at least raise an error if a new sample has column names that are missing, or if it has extra columns. No need to check for the type for now.

I'm in favor of having an error especially because we want to avoid silent issues as much as possible - i.e. when something goes wrong (when schemas don't match or some data are missing) and no errors/warnings are raised.

Consistency between streaming and non-streaming is also important.