Macro decl_ssd_autodiff_backend_ext 

macro_rules! decl_ssd_autodiff_backend_ext {
    ($autodiff_trait:ident, $ext_trait:path $(, $extra_bound:path)*) => { ... };
}

Declare a marker trait $autodiff_trait: Backend + $ext_trait + AutodiffBackend and a blanket impl for burn::backend::Autodiff<B> whenever B: $ext_trait.

Both Mamba-2 and Mamba-3 expose this marker to let callers bound their own generics on “autodiff-capable backend that also implements the SSD ext.”