trainBatch
no need in flatten because classifier expect the arrays of labels for every classifier.
['Accept', 'Offer']

toFormat
we need to get the array of all relevant labels.
for example this is what we get 
[ 'Offer',
  'Job Description',
  'Promotion Possibilities',
  'Job Description:QA',
  'Job Description:Team Manager',
  'Promotion Possibilities:Fast promotion track' ]