add_requirement can't be called before the output is defined

[abadams]

If you put an add_requirement at the top of the generate method, you get the very confusing error "Output "foo" was not defined."

Took me a while to figure out what was wrong. We should either give a better error, or make it possible to add requirements earlier.

[UncleHandsome]

Hello Adams
Not only add_requirement, but all the use before output is defined will fail.
For example, I tried to pad input by output's bound.
Ex: Func padded_input = repeat_edge(input, {{0, output.width()}, {0, output.height}});
This will encounter the same error.

I tried quick fix to avoid this problem.
It works temporally, but not sure it is correct or not.

vector<OutputImageParam> Func::output_buffers() const {
    if (!defined()) {
        Parameter output(Handle(), true, 4, name());
        return {OutputImageParam(output, Argument::OutputBuffer, *this)};
    }

[steven-johnson]

Making add_requirements() allow for this would be a bit tricky. Making the error message clearer would be easy.