Function wrap_burn 

pub fn wrap_burn<B, M, O>(
    inner: O,
    learning_rate: f64,
) -> BurnOptimizer<B, M, O>
where
    B: AutodiffBackend,
    M: AutodiffModule<B>,
    O: Optimizer<M, B>,

Helper: wrap a freshly-built Burn optimizer in a BurnOptimizer.