Compute accuracy after expanding `y_true` to the size of `y_pred`.

def accuracy_thresh_expand(y_pred:Tensor, y_true:Tensor, thresh:float=0.5, sigmoid:bool=True)->Rank0Tensor:
    "Compute accuracy after expanding `y_true` to the size of `y_pred`."
    if sigmoid: y_pred = y_pred.sigmoid()
    return ((y_pred>thresh)==y_true[:,None].expand_as(y_pred).byte()).float().mean()