For data-only datasets, streaming and non-streaming don't behave the same
Hi @severo, thanks for reporting.

Yes, this happens because when non-streaming, a cast of all data is done in order to "concatenate" it all into a single dataset (thus the error), while this casting is not done while yielding item by item in streaming mode.

Maybe in streaming mode we should keep the schema (inferred from the first item) and throw an exception if a subsequent item does not conform to the inferred schema?
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
``` 
 Hi @severo, thanks for reporting.

Yes, this happens because when non-streaming, a cast of all data is done in order to "concatenate" it all into a single dataset (thus the error), while this casting is not done while yielding item by item in streaming mode.

Maybe in streaming mode we should keep the schema (inferred from the first item) and throw an exception if a subsequent item does not conform to the inferred schema?