text: Title: Option to run PretrainedTransformer in eval mode
  Body: While setting `train_parameters` to `False` very often we also may consider disabling dropout/batchnorm, in other words, to run the pretrained model in eval mode.
  We've done a little modification to `PretrainedTransformerEmbedder` that allows providing whether the token embedder should be forced to `eval` mode during the training phase.
  
  Do you this feature might be handy? Should I open a PR?
  