RNG should not be static

[laggui]

Issue initially raised in #2923

burn/crates/burn-ndarray/src/backend.rs

Line 11 in e53978f

pub(crate) static SEED: Mutex<Option<StdRng>> = Mutex::new(None);

burn/crates/burn-cubecl/src/backend.rs

Line 12 in e53978f

pub(crate) static SEED: Mutex<Option<StdRng>> = Mutex::new(None);

After some discussions, the RNG should probably be per-device.

pub trait Backend {
    /// Device type.
    type Device: DeviceOps;

    /// Seed the backend device.
    fn seed(seed: u64, device: &Self::Device);
}

For ndarray specifically, given that the only device is CPU we can probably just make sure it is thread-local

thread_local! {
    pub(crate) static SEED: RefCell<Option<StdRng>> = RefCell::new(None);
}