Move fakedata generation to separate module

[pmeier]

This moves the fakedata generation to aseparate module as discussed in #1010. I furthermore renamed get_mnist_data to mnist_root to match the style of CIFAR and ImageNet.

[codecov-io]

Codecov Report

Merging #1014 into master will decrease coverage by 0.07%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1014      +/-   ##
==========================================
- Coverage   63.24%   63.17%   -0.08%     
==========================================
  Files          65       65              
  Lines        5148     5148              
  Branches      772      772              
==========================================
- Hits         3256     3252       -4     
- Misses       1669     1672       +3     
- Partials      223      224       +1

Impacted Files	Coverage Δ
torchvision/transforms/functional.py	69.9% <0%> (-1.22%)	⬇️

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[fmassa]

Awesome, thanks a lot!