Fixes observer attachment to model based on config for wanda sparsifi…

…er (pytorch#1265)

* Fixes observer attachment to model based on config for wanda sparsifier

* handles case when no config is specified

* Added test case in test_wanda.py for custom config

* lint fix

test/sparsity/test_wanda.py

@@ -113,6 +113,39 @@ def test_two_layer_mlp_unstructured(self):
 
         sparsifier.squash_mask()
 
+    def test_two_layer_mlp_unstructured_custom_config(self):
+        model = nn.Sequential(
+            nn.Linear(128, 200), nn.ReLU(), nn.Linear(200, 10)
+        )  # C_in by C_out
+        X1 = torch.randn(100, 128)  # B1 by C_in
+        X2 = torch.randn(50, 128)  # B2 by C_in
+
+        # Define custom config to sparsify only the first Linear layer for testing
+        config = [{"tensor_fqn": "0.weight"}]
+
+        sparsifier = WandaSparsifier(sparsity_level=0.5)
+        sparsifier.prepare(model, config=config)
+
+        model(X1)
+        model(X2)
+        sparsifier.step()
+
+        cnt = 0
+        for m in model.modules():
+            if isinstance(m, nn.Linear):
+                cnt += 1
+                sparsity_level = (m.weight == 0).float().mean()
+                if cnt == 1:  # First Linear layer should have 50% sparsity
+                    assert (
+                        sparsity_level == 0.5
+                    ), f"sparsity for linear layer {cnt} should be 0.5"
+                else:  # Other layers should not be sparsified
+                    assert (
+                        sparsity_level != 0.5
+                    ), f"sparsity for linear layer {cnt} should not be 0.5"
+
+        sparsifier.squash_mask()
+
 
 if __name__ == "__main__":
     unittest.main()

torchao/sparsity/wanda.py

@@ -3,7 +3,7 @@
 
 import torch
 from torch import nn
-from torch.ao.pruning import BaseSparsifier
+from torch.ao.pruning import BaseSparsifier, get_arg_info_from_tensor_fqn
 from torch.ao.quantization import QConfig, default_placeholder_observer
 from torch.ao.quantization.quantize import _remove_qconfig
 
@@ -47,9 +47,27 @@ def __init__(
     def prepare(self, model: nn.Module, config: List[Dict]) -> None:
         # activation: use PerChannelNormObserver
         # use no-op placeholder weight observer
-        model.qconfig = QConfig(
-            activation=PerChannelNormObserver, weight=default_placeholder_observer
-        )  # type: ignore[assignment]
+        if config is None:
+            # If no config is provided, apply the qconfig to the entire model
+            model.qconfig = QConfig(
+                activation=PerChannelNormObserver, weight=default_placeholder_observer
+            )  # type: ignore[assignment]
+        else:
+            for module_config in config:
+                tensor_fqn = module_config.get("tensor_fqn", None)
+                if tensor_fqn is None:
+                    raise ValueError("Each config must contain a 'tensor_fqn'.")
+
+                # Extract module information from tensor_fqn
+                info_from_tensor_fqn = get_arg_info_from_tensor_fqn(model, tensor_fqn)
+                module = info_from_tensor_fqn["module"]
+
+                # Apply the qconfig directly to the module if it exists
+                if module is not None:
+                    module.qconfig = QConfig(
+                        activation=PerChannelNormObserver,
+                        weight=default_placeholder_observer,
+                    )  # type: ignore[assignment]
         torch.ao.quantization.prepare(model, inplace=True)
 
         # call superclass prepare