Move initializers to WeightInitializers

[MartinuzziFrancesco]

Following issue 21 in WeightInitializers.jl we should do something similar for the initializers that we have here

[avik-pal]

For centralized maintenance, I think it makes sense to move those initializers to WeightInitializers as well?

[MartinuzziFrancesco]

that's a good point, I'll finish dealing with issue 21 and then we can move them there