[ao][sparsity] Implementation of register_layer() and get_mask() (#80887)
The register_layer() attaches a pre-forward hook to the layer to aggregate
activations over time. The mask shape is also inferred here.
The get_mask() returns the computed mask associated to the attached layer.
The mask is
- a torch tensor is features for that layer is None.
- a list of torch tensors for each feature, otherwise
Test Plan:
```python test/test_ao_sparsity.py TestActivationSparsifier```
Pull Request resolved: https://github.com/pytorch/pytorch/pull/80887
Approved by: https://github.com/z-a-f
