Fix typo in __init__.py: 'all' variable should be '__all__'

**Describe the bug**
In `gsplat/__init__.py`, the list of exported modules is assigned to the variable `all` instead of the standard `__all__`.

**To Reproduce**
Check line 34 of `gsplat/__init__.py`.

```python
# Current code
all = [
    "PngCompression",
    ...
]

# Fixed code
__all__ = [
    "PngCompression",
    ...
]