[ao][sparsity] Implementation of squash_mask() (#80889)
Unregisters aggreagate hook that was applied earlier and registers sparsification hooks.
The sparsification hook will apply the mask to the activations before it is fed into the
attached layer.
Test Plan:
```python test/test_ao_sparsity.py TestActivationSparsifier```
Pull Request resolved: https://github.com/pytorch/pytorch/pull/80889
Approved by: https://github.com/z-a-f
