[dattri.model_utils] Add a wrapper for register_forward_hook

[tingwl0122]

Description

This PR implemented a wrapper function to enable register_forward_hook on users' model (at a specific location).

1. Motivation and Context

For the Representer Point Selection algorithm, we will want to use the last intermediate layer's feature to compute the representer score for explainability. This hook can help users get the desired layer's feature (not restricted to the last intermediate) after forward-passing their data to their model once.

2. Summary of the change

• add one hook.py file containing a single function get_feature.
• add one test file named test_hook.py

3. What tests have been added/updated for the change?

• [x] Unit test: Typically, this should be included if you implemented a new function/fixed a bug.

[tingwl0122]

Hi @TheaperDeng, please have a look if you have time.

[tingwl0122]

The hook function will be deprecated in the #58. The current get_layer_feature will extract the output of the second-to-last layer, which ignores possible activation functions applied afterward. Hence, we just directly obtain the input of the last layer.