MNIST loader refactored: permanent 'data' and 'targets' fields

[dizcza]

Addresses #577.
Simplified code.

[fmassa]

LGTM, thanks!

[lanpa]

Hi, This PR breaks many existing example which using MNIST and it might be hard to trace the root cause for people unfamiliar with pytorch. To be more user friendly, I suggest to add those test_data, test_label etc. (as alias) back along with deprecation message before publishing the next pip release.

[dizcza]

Yes, alias with a deprecation message, saying it'll be removed in the next release, might be a good idea.
What do you think, @fmassa ?

[fmassa]

@dizcza I agree with this path.
I was planning on cutting a release this week, would it be possible to get this deprecation message in soon?

Thanks!

[dizcza]

I won't be available this week. Please find someone else.

[fmassa]

Sounds good, I'll get this done