Implement OrthogonalInitializer

[takuseno]

Hi, @TE-TakuyaNarihira san, @TE-AkioHayakawa san!
#243 takes a bit time because my school started 😢

Anyway, I implemented OrthogonalInitializer which is widely available on other DNN libraries but still effective way in many domains.

If you think it might be good to nnabla, I will test this more to remove WIP sign.

Thank you!

[takuseno]

PS. Are there C++ initialization codes? I could not find any files related to weight initialization.

[TakuyaNarihira]

Thanks for the PR.

PS. Are there C++ initialization codes? I could not find any files related to weight initialization.

No C++ initialization so far. All weight initialization is performed in Python-side.

[TakuyaNarihira]

@takuseno I'll review soon.

[TakuyaNarihira]

It must be useful for many people, and the code logic looks good to me. Thanks for your PR. Could you add unit tests for this feature? See the unit testing script for initializers which we just added at #270 for this PR :).

[takuseno]

@TE-TakuyaNarihira Thank you! I'll handle that!

[takuseno]

@TE-TakuyaNarihira
I've added the unit test!! It is based on PyTorch implementation. And, the test has passed on my machine.

[takuseno]

@TE-KazukiYoshiyama san. I set a default rng with RandomState(313), and I confirmed that my unit test has passed.

[AkioHayakawa-sony]

This PR is moved to #547 and will be merged to master.