Function access_all_4 

pub fn access_all_4<A, B, C, D>(
    a: &GpuRef<A>,
    b: &GpuRef<B>,
    c: &GpuRef<C>,
    d: &GpuRef<D>,
) -> Result<(Arc<CudaSlice<A>>, Arc<CudaSlice<B>>, Arc<CudaSlice<C>>, Arc<CudaSlice<D>>), GpuError>

Validate four input GpuRefs and return owning Arcs. Used by cuDNN convolution which takes (input, filter, output, workspace).