Tuning BERT on our own data set for multi-class classification problem
I want to tune pre-trained BERT for multi-class classification with **6 million class, 30 million rows & highly imbalance data set.**  
Can we tune BERT in batch of classes? 
For example, I will take 15 classes (last layer will have only 15 neuron) and train my BERT model & in next batch use that trained model to train batch of next 15 classes, I just want to understand cons of this process.
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