[sparsity] Change API to take FQNs as configuration (#65296)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/65296
The original API described in the https://github.com/pytorch/pytorch/issues/59835
assumed that the per-layer configuration would take a module/layer
reference. However, a more useful approach is to refer to the layers
by their fully qualified names (FQN). That allows us to store the
configuration in a file without serializing the models.
We define a layer's FQN as it's "path" within a model. For example,
if one can refer to a model using `model.layer0.sublayerX`, the FQN
of the sublayerX is `'layer0.sublayerX'`.
Test Plan:
```
python test/test_ao_sparsity.py -- TestBaseSparsifier
buck test mode/opt //caffe2:test -- TestBaseSparsifier
```
Reviewed By: gchanan
Differential Revision: D31186830
Pulled By: z-a-f
fbshipit-source-id: d8d87f1c054e5c10d470e67837476a11e0a9b1d4
