`add_faiss_index` raises ValueError: not enough values to unpack (expected 2, got 1)
### Describe the bug

Calling `add_faiss_index` on a `Dataset` with a column argument raises a ValueError. The following is the trace

```python
    214 def replacement_add(self, x):
    215     """Adds vectors to the index.
    216     The index must be trained before vectors can be added to it.
    217     The vectors are implicitly numbered in sequence. When `n` vectors are
   (...)
    224         `dtype` must be float32.
    225     """
--> 227     n, d = x.shape
    228     assert d == self.d
    229     x = np.ascontiguousarray(x, dtype='float32')

ValueError: not enough values to unpack (expected 2, got 1)
```

### Steps to reproduce the bug

1. Load any dataset like `ds = datasets.load_dataset("wikimedia/wikipedia", "20231101.en")["train"]`
2. Add an FAISS index on any column `ds.add_faiss_index('title')`

### Expected behavior

The index should be created

### Environment info

- `datasets` version: 2.18.0
- Platform: Linux-6.5.0-26-generic-x86_64-with-glibc2.35
- Python version: 3.9.19
- `huggingface_hub` version: 0.22.2
- PyArrow version: 15.0.2
- Pandas version: 2.2.1
- `fsspec` version: 2024.2.0
- `faiss-cpu` version: 1.8.0