[ao][sparsity] Implementation of step() and update_mask() (#80888)
The step() internally calls the update_mask() function for each layer
The update_mask() applies reduce_fn and mask_fn to compute the sparsification mask.
Note:
the reduce_fn and mask_fn is called for each feature, dim over the data
Test Plan:
```python test/test_ao_sparsity.py TestActivationSparsifier```
Pull Request resolved: https://github.com/pytorch/pytorch/pull/80888
Approved by: https://github.com/z-a-f
