code: @Override public void init(boolean expensive) {
      super.init(expensive); //drop constant and ignored columns
      _parms.validate(this, expensive);
      if (expensive && error_count() == 0) checkMemoryFootPrint();
    }
docs: Initialize the ModelBuilder, validating all arguments and preparing the
  training frame.  This call is expected to be overridden in the subclasses
  and each subclass will start with "super.init();".  This call is made
  by the front-end whenever the GUI is clicked, and needs to be fast;
  heavy-weight prep needs to wait for the trainModel() call.
  
  Validate the very large number of arguments in the DL Parameter directly.