How can I build a comprehensive deep learning model using PyTorch that integrates two types of data: $3$D medical imaging data and clinical data? Specifically, I want to create a CNN with attention layers to process the $3$D image data, and a DNN to handle the clinical data. The model should combine these two pathways and provide a unified output. Could you provide detailed code and explain how each part of the architecture works, including the integration of the CNN$,$ attention mechanism, and DNN$?$