Data alteration when loading dataset with unspecified inner sequence length
### Describe the bug

When a dataset saved with a specified inner sequence length is loaded without specifying that length, the original data is altered and becomes inconsistent.

### Steps to reproduce the bug

```python
from datasets import Dataset, Features, Value, Sequence, load_dataset

# Repository ID
repo_id = "my_repo_id"

# Define features with a specific length of 3 for each inner sequence
specified_features = Features({"key": Sequence(Sequence(Value("float32"), length=3))})

# Create a dataset with the specified features
data = [
    [[1.0, 2.0, 3.0], [4.0, 5.0, 6.0]],
    [[7.0, 8.0, 9.0], [10.0, 11.0, 12.0]],
]
dataset = Dataset.from_dict({"key": data}, features=specified_features)

# Push the dataset to the hub
dataset.push_to_hub(repo_id)

# Define features without specifying the length
unspecified_features = Features({"key": Sequence(Sequence(Value("float32")))})

# Load the dataset from the hub with this new feature definition
dataset = load_dataset(f"qgallouedec/{repo_id}", split="train", features=unspecified_features)

# The obtained data is altered
print(dataset.to_dict())  # {'key': [[[1.0], [2.0]], [[3.0], [4.0]]]}
```


### Expected behavior

```python
print(dataset.to_dict())  # {'key': [[[1.0, 2.0, 3.0], [4.0, 5.0, 6.0]], [[7.0, 8.0, 9.0], [10.0, 11.0, 12.0]]]}
```

### Environment info

- `datasets` version: 2.14.4
- Platform: Linux-6.2.0-32-generic-x86_64-with-glibc2.35
- Python version: 3.9.12
- Huggingface_hub version: 0.15.1
- PyArrow version: 12.0.1
- Pandas version: 2.0.3