[Code scan] Fix invalid dftio.data __all__ exports

This issue comes from a Codex global repository scan.

## Problem
`dftio.data.__all__` contains imported objects instead of string names:

https://github.com/deepmodeling/dftio/blob/c9d128f24a74ef2911e1a28f5640357488beb196/dftio/data/__init__.py#L15-L29

Python expects every `__all__` entry to be a string. As written, wildcard imports fail instead of exporting the public API.

## Reproduction
Run:

```python
from dftio.data import *
```

This raises `TypeError: Item in dftio.data.__all__ must be str`.

## Suggested fix
Change entries to string names and remove the duplicate `feature_to_block` entry.