FEATURE: possibility to define global Classload for ND4J

[hosuaby]

This PR is related to #8712. This issue is talking about the need to have a possibility to provide class loader for method Class.forName. Author mention only class WordVectorSerializer from deeplearning-nlp. But the problem is more general. We need to have possibility to chose classloader used by ND4J and Deeplearning4j.

This PR is a first part of the work. I added a new class ND4JClassLoading that gives possibility to used to provide his own classloader used by Class.forName and ServiceLoader.load. And I replaced all call to those methods by calls to this new class. I hope I forgot nothing.

There will be a second part of this work in anouther PR, concerning Dl4j. Both PR will be inderpendent.

[agibsonccc]

@hosuaby hey sorry about this. I'm just getting to some of these, do you mind updating? I understand if you can't, otherwise we can get this merged.

[agibsonccc]

Hey, sorry this is late. Could you modify it to use the nd4j class loader? Otherwise, this seems legit. Sorry for the late response!

[hosuaby]

Hello, @agibsonccc :)
Thanks for review. First, I made a rebase of branch and solved conflicts with current master.
I don't understood your demend. If need any modification, don't hesitate to ask. What exactly should I modify ?

[hosuaby]

Hello, @agibsonccc .

I think I undestood what you wanna say. My mistake, I have better to explain my plan.

This PR is the first part of work concerning #8712. It provides ability to define classloader for ND4J. For DL4J I would like to have a separate class DL4JClassLoading, who will behaivour the same way, but for Dl4j packages.

The reasons I wanna to keep two classloaders separate:

1. Nd4j and Dl4j are semantically two distinct projects
2. In my use case, I only need to redefine classloader for ND4j.
3. If I decide to rely on ND4JClassLoading from dl4j packages, it's will not be clear for user why should he define classloader for nd4j instead of dl4j.

So, my plan is, if this PR is approved, I make immediately the second one with DL4JClassLoading class.

Tell me what you think about.

[hosuaby]

Hello, @agibsonccc
Any updates about this PR ?

[agibsonccc]

Minor nit

[agibsonccc]

@hosuaby minor nit then I think we can merge this. Thanks!

[hosuaby]

@agibsonccc Ok. Do not hesitate to ask any change!

[agibsonccc]

@hosuaby think you could get this over the line? I think we can merge this soon.

[hosuaby]

@agibsonccc done

[agibsonccc]

Great thanks @hosuaby I want to ping @saudet for 1 review here. @saudet do you see any major issues here?

[saudet]

I didn't test it, but it looks fine.

[hosuaby]

@agibsonccc Thanks!
Now I will take care of #8712 that is very similar.