Just to understand your question correctly, you would like to use a pre-trained model e.g. resnet as a feature extractor in your main model.
In your use case it’s not sufficient to get the last activation from the feature extractor, but you would like to get all activations from all blocks? Since these “blocks” have different output shapes, you would like to fix them somehow. Is this understanding correct?
Could you explain a bit more about the activations, i.e. what would you like to do with them? Feeding them into another model?