R2 score (coefficient of determination) between `pred` and `targ`.

def r2_score(pred:Tensor, targ:Tensor)->Rank0Tensor:
    "R2 score (coefficient of determination) between `pred` and `targ`."
    pred,targ = flatten_check(pred,targ)
    u = torch.sum((targ - pred) ** 2)
    d = torch.sum((targ - targ.mean()) ** 2)
    return 1 - u / d