Added support for the QMNIST dataset

[leonbottou]

QMNIST https://github.com/facebookresearch/qmnist

Patch

The patch has two commits.

• First commit adds a function read_sn3_pascalvincent_tensor that reads all variants of the mnist file format. Can read compressed files. Can accept a file object instead of a filename. Redefine functions read_label_file and read_image_file using this generic function.
• The second commit adds class QMNIST. Usage is the same as the qmnist.py loader provided with the QMNIST distribution. An interesting aspect is the argument 'what' that specifies which subset to access: 'train', 'test', 'test10k', 'test60k', 'nist'. When argument what is absent, the 'train' argument is consulted.

Remarks.

The way I handle compressed files in read_sn3_pascalvincent_tensor does not square well with the new utility function download_and_extract_archive. My take is that this function should only deal with archives (zip, tar, tgz) but not compressed files (gz, xz). In the end, the QMNIST class uses download_url instead.

Instead the QMNIST class uses download_url. My take is that the

[leonbottou]

Note: With python-2.7, there is no lzma module by default.
One has to use https://anaconda.org/conda-forge/backports.lzma.
I do not know how to get Travis to do this for me...

[fmassa]

Thanks a lot for the PR Leon!

I've made a few minor comments. I believe we can fix CI by moving the import of lzma inside the function call.

Let me know if you want me to address the comments that I've made.

[codecov-io]

Codecov Report

Merging #995 into master will decrease coverage by 0.14%.
The diff coverage is 38.63%.

@@            Coverage Diff             @@
##           master     #995      +/-   ##
==========================================
- Coverage   62.69%   62.54%   -0.15%     
==========================================
  Files          65       65              
  Lines        5080     5148      +68     
  Branches      761      772      +11     
==========================================
+ Hits         3185     3220      +35     
- Misses       1678     1711      +33     
  Partials      217      217

Impacted Files	Coverage Δ
torchvision/datasets/mnist.py	50.71% <38.63%> (-12.53%)	⬇️
torchvision/datasets/utils.py	58.51% <0%> (-2.97%)	⬇️
torchvision/models/mobilenet.py	89.7% <0%> (-2.61%)	⬇️
torchvision/datasets/celeba.py	17.24% <0%> (+1.14%)	⬆️
torchvision/models/densenet.py	86.79% <0%> (+1.56%)	⬆️
torchvision/models/segmentation/segmentation.py	67.44% <0%> (+25.18%)	⬆️

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[fmassa]

Thanks a lot for the PR Leon!