sketch_rnn: allow loading of multiple datasets (#672)
sketch_rnn: allow loading of multiple datasets (#672)

This allows training on the concatentation of multiple
datasets. This is done by passing an array in the existing
hparams.data_set field. Change was made backwards compatible.
jesseengel/magenta,jesseengel/magenta,magenta/magenta,adarob/magenta,adarob/magenta,magenta/magenta