add image-recordio-gen cmd and RecordIO reader

[QiJune]

The image-recordio-gen command converts an image folder with label txt to recordio file format.

Let's take mnist dataset as an example. We could download the dataset from https://github.com/myleott/mnist_png.git.

The dataset contains two directories: training and testing. We need to make a label file. The label file maps a class string to an int index. Following is the label file for mnist dataset.

0
1
2
3
4
5
6
7
8
9

Then, we could run the image-recordio-gen command:

$GOPATH/bin/image-recordio-gen -label=$MNIST/label.txt -dataset=$MNIST/training -output=$MNIST/train_record -recordsPerShard=1500

We could find the recordio shard files in train_record directory:

data-00000
data-00001
...
...

[codecov]

Codecov Report

Merging #377 (58f47af) into develop (9afed2f) will decrease coverage by 0.51%.
The diff coverage is 64.86%.

@@             Coverage Diff             @@
##           develop     #377      +/-   ##
===========================================
- Coverage    87.77%   87.25%   -0.52%     
===========================================
  Files           33       35       +2     
  Lines         1505     1538      +33     
===========================================
+ Hits          1321     1342      +21     
- Misses         121      129       +8     
- Partials        63       67       +4     

Impacted Files	Coverage Δ
tool/recordio/imagerecord.go	33.33% <33.33%> (ø)
vision/imageloader/imageloader.go	91.15% <66.66%> (-0.06%)	⬇️
tool/recordio/reader.go	76.00% <76.00%> (ø)