[ao][sparsity] Store mask as sparse coo during serialization for the activation sparsifier (#82181)
The stored mask is dumped as `torch.sparse_coo` while serializing. While restoring the state,
the mask is converted to a dense tensor again.
Test Plan:
```python test/test_ao_sparsity.py TestActivationSparsifier```
Pull Request resolved: https://github.com/pytorch/pytorch/pull/82181
Approved by: https://github.com/z-a-f
