Fix stubgen

[zcbenz]

After bumping version of nanobind with #2896 the structure of type stubs have changed.

Before:

python/mlx/core
├── __init__.pyi
├── cuda
│   └── __init__.pyi
├── distributed
│   └── __init__.pyi
├── fast
│   └── __init__.pyi
├── fft
│   └── __init__.pyi
├── linalg
│   └── __init__.pyi
├── metal
│   └── __init__.pyi
└── random
    └── __init__.pyi

After:

python/mlx/core
├── __init__.pyi
└── core
    ├── __init__.pyi
    ├── cuda.pyi
    ├── distributed.pyi
    ├── fast.pyi
    ├── fft.pyi
    ├── linalg.pyi
    ├── metal.pyi
    └── random.pyi

Which was caused by wjakob/nanobind#892. With this PR the structure will become:

python/mlx/core
├── __init__.pyi
├── cuda.pyi
├── distributed.pyi
├── fast.pyi
├── fft.pyi
├── linalg.pyi
├── metal.pyi
└── random.pyi

[angeloskath]

Awesome thanks!