Fix channels in encoded Datum
Fix channels in encoded Datum

Encoded Datum objects can have the data shape specified through `width`, `height` and `channels` fields.
DIGITS wrappers for Torch relies on this to determine the shape of the input data.
The `channel` field was previously left uninitialized.
This change fixes this.

Deepomatic/DIGITS,TimZaman/DIGITS,dongjoon-hyun/DIGITS,TimZaman/DIGITS,jmancewicz/DIGITS,Lucaszw/DIGITS,gheinrich/DIGITS-GAN,TimZaman/DIGITS,Deepomatic/DIGITS,Deepomatic/DIGITS,jmancewicz/DIGITS,bygreencn/DIGITS,ethantang95/DIGITS-GAN,ethantang95/DIGITS,ethantang95/DIGITS-GAN,bygreencn/DIGITS,dongjoon-hyun/DIGITS,Deepomatic/DIGITS,ethantang95/DIGITS-GAN,jmancewicz/DIGITS,ethantang95/DIGITS,TimZaman/DIGITS,gheinrich/DIGITS-GAN,ethantang95/DIGITS-GAN,dongjoon-hyun/DIGITS,jmancewicz/DIGITS,gheinrich/DIGITS-GAN,bygreencn/DIGITS,dongjoon-hyun/DIGITS,Lucaszw/DIGITS,ethantang95/DIGITS,gheinrich/DIGITS-GAN,Lucaszw/DIGITS,bygreencn/DIGITS,ethantang95/DIGITS,Lucaszw/DIGITS