New style square function

[kmaehashi]

This PR implements new style square function.

I also fixed unary_math_function_test decorator to support testing FunctionNode.
I think this is transitional; once #3499 got merged, it is better to rewrite tests with the new function test template.

[takagi]

I got the situation. We don't need to keep unary_math_function_test to accept Function class once we will have moved to new style functions.

Actually the number of places where the decorator is now used is only five, so would you make the decorator accept only FunctionNode class with having some working around just to keep CI pass, for example, temporarily leaving old unary_math_function_test for Function class with renamed and removing it after we have finished to move to new style functions.

I suppose it would be enough.

[takagi]

We should follow API compat policy for unary_math_function_test, so my previous comment does not suit.

[kmaehashi]

Ok, thank you for confirmation!

[takagi]

It would be more natural for Chainer to let unary_math_function_test take a Python function instead of a FunctionNode object as @beam2d mentioned on slack.

master...takagi:fix-test-math-function

Here unary_math_function_test accept a Function object just for backward compatibility and we would be able to deprecate it in the future major bump. (I will update the docstring shortly)

How about this?

[kmaehashi]

Thank you for the suggestion, it looks good to me.

[takagi]

I posted a PR to fix unary_math_function_test to support new style functions as #3603. Would you rebase your change on top of it after it will have been merged into the master? Before that, you can leave this PR.

[kmaehashi]

Rebased.

[takagi]

LGTM!