[ao][sparsity] Post training data sparsifier callback for lightning (#80370)
Lightning callback that enables post-training sparsity.
This callback aims to sparsify the model inside lightning module after training.
**Note that the model is copied and then sparsified, so the existing model is not modified**
The sparsified model can be used for comparison and can be accessed using <callback_obj>.sparsified
Test Plan
```python torch/ao/sparsity/_experimental/data_sparsifier/lightning/tests/test_callbacks.py TestPostTrainingCallback```
Pull Request resolved: https://github.com/pytorch/pytorch/pull/80370
Approved by: https://github.com/z-a-f
