Partially execute a model:

Sometimes you may want to obtain an intermediate tensor inside a model, such as the input of certain layer, the output before post-processing. Since there are typically hundreds of intermediate tensors, there isn’t an API that provides you the intermediate result you need. You have the following options:

1. Write a (sub)model. Following the tutorial, you can rewrite a model component (e.g. a head of a model), such that it does the same thing as the existing component, but returns the output you need.

2. Partially execute a model. You can create the model as usual, but use custom code to execute it instead of its forward(). For example, the following code obtains mask features before mask head.

   images = ImageList.from_tensors(...)  # preprocessed input tensor
   model = build_model(cfg)
   model.eval()
   features = model.backbone(images.tensor)
   proposals, _ = model.proposal_generator(images, features)
   instances, _ = model.roi_heads(images, features, proposals)
   mask_features = [features[f] for f in model.roi_heads.in_features]
   mask_features = model.roi_heads.mask_pooler(mask_features, [x.pred_boxes for x in instances])
   

   　　

3. Use forward hooks. Forward hooks can help you obtain inputs or outputs of a certain module. If they are not exactly what you want, they can at least be used together with partial execution to obtain other tensors.

All options require you to read documentation and sometimes code of the existing models to understand the internal logic, in order to write code to obtain the internal tensors.