get F input dimension

function intrusive_input_dim = get_F_input_dim(F)

d = length(F) ;

j = 1;

intrusive_input_dim = [];
while isempty(intrusive_input_dim) || j <= (d)

    if ~isempty(F{j}) && nnz(F{j})

    intrusive_input_dim = size(F{j},2);

    end

    j=j+1;

end

if isempty(intrusive_input_dim)
    error("Failed to set input dimension of nonlinearity")
end
end