I've been trying to use your example using BERT for tweets classification (simple Kaggle dataset)

trainembeddings =    NetModel["BERT Trained on BookCorpus and English Wikipedia Data"][
    forBERT[[All, 1]], TargetDevice -> "CPU"] -> forBERT[[All, 2]];

classifierhead = NetChain[{DropoutLayer[], NetMapOperator[2], 
                                       AggregationLayer[Max, 1], SoftmaxLayer[]}, 
                                      "Output" -> NetDecoder[{"Class", {"negative", "positive"}}]]
bertresults = NetTrain[classifierhead, trainembeddings, All, TargetDevice -> "CPU",
   MaxTrainingRounds -> 50]

But, after I build the model, I've no idea how to score with new tweets, and I cannot see how to do it in your example.