Add support for captum toplevel-import.

[dkrako]

This fixes issue #680 Strange import issue --> AttributeError: module 'captum' has no attribute 'attr'

In most python packages, you can import the toplevel package, like numpy, scipy, torch, etc.. and then access the submodules simply by the dot-operator. Like you can use import numpy and after that you can use any submodules à la numpy.random.uniform.

With this PR, you can just import captum and then for example use captum.attr.DeepLift or captum.robust.Perturbation instead of having to import both. It's just a small convenience, and I think there are more people that expect this kind of import to work but don't bother to create an issue out of this.

I hope this PR is considered as helpful.

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[NarineK]

Thank you for the PR, @dkrako! @aobo-y, @vivekmig do you mind checking whether there is import runtime performance related issues with this change ?

[vivekmig]

Thanks so much for adding this @dkrako !

Thank you for the PR, @dkrako! @aobo-y, @vivekmig do you mind checking whether there is import runtime performance related issues with this change ?

This seems fine, import captum is slower, but comparable to importing captum.attr currently, so should be fine considering the benefits of having this import.

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