Fix state dict loading in bitsandbytes plugin when checkpoint is already quantized

src/lightning/fabric/CHANGELOG.md

@@ -50,7 +50,7 @@ The format is based on [Keep a Changelog](http://keepachangelog.com/en/1.0.0/).
 
 ### Fixed
 
--
+- Fixed a matrix shape mismatch issue when running a model loaded from a quantized checkpoint (bitsandbytes) ([#19886](https://github.com/Lightning-AI/lightning/pull/19886))
 
 -
 

src/lightning/fabric/plugins/precision/bitsandbytes.py

@@ -233,9 +233,9 @@ def quantize_(self, weight: Optional[torch.Tensor] = None, device: Optional[torc
             """Inplace quantize."""
             if weight is None:
                 weight = self.weight.data
-                if weight.data.type == torch.int8:
-                    # already quantized
-                    return
+            if weight.data.dtype == torch.int8:
+                # already quantized
+                return
             assert isinstance(self.weight, bnb.nn.Int8Params)
             self.weight = self.quantize(self.weight, weight, device)
 
@@ -317,9 +317,9 @@ def quantize_(self, weight: Optional[torch.Tensor] = None, device: Optional[torc
             """Inplace quantize."""
             if weight is None:
                 weight = self.weight.data
-                if weight.data.type == torch.uint8:
-                    # already quantized
-                    return
+            if weight.data.dtype == torch.uint8:
+                # already quantized
+                return
             assert isinstance(self.weight, bnb.nn.Params4bit)
             self.weight = self.quantize(self.weight, weight, device)
 

src/lightning/pytorch/CHANGELOG.md

@@ -45,7 +45,7 @@ The format is based on [Keep a Changelog](http://keepachangelog.com/en/1.0.0/).
 
 ### Fixed
 
--
+- Fixed a matrix shape mismatch issue when running a model loaded from a quantized checkpoint (bitsandbytes) ([#19886](https://github.com/Lightning-AI/lightning/pull/19886))
 
 
 - Fixed `WandbLogger.log_hyperparameters()` raising an error if hyperparameters are not JSON serializable ([#19769](https://github.com/Lightning-AI/pytorch-lightning/pull/19769))

tests/tests_fabric/plugins/precision/test_bitsandbytes.py

@@ -230,3 +230,37 @@ def __init__(self):
     assert not keys.missing_keys
     assert model.l.weight.device.type == "cuda"
     assert model.l.weight.dtype == expected
+
+
+@RunIf(min_cuda_gpus=1, min_torch="2.1")
+@pytest.mark.skipif(not _BITSANDBYTES_AVAILABLE, reason="bitsandbytes unavailable")
+def test_load_quantized_checkpoint(tmp_path):
+    """Test that a checkpoint saved from a quantized model can be loaded back into a quantized model."""
+
+    class Model(torch.nn.Module):
+        def __init__(self):
+            super().__init__()
+            self.linear = torch.nn.Linear(16, 16, bias=False)
+
+        def forward(self, x):
+            return self.linear(x)
+
+    fabric = Fabric(accelerator="cuda", devices=1, plugins=BitsandbytesPrecision("nf4-dq"))
+    model = Model()
+    model = fabric.setup(model)
+    model(torch.randn(2, 16, device=fabric.device))
+    state_dict = model.state_dict()
+    # The checkpoint contains quantized weights
+    assert state_dict["linear.weight"].dtype == torch.uint8
+    assert state_dict["linear.weight"].shape == (128, 1)
+    torch.save(state_dict, tmp_path / "checkpoint.pt")
+
+    fabric = Fabric(accelerator="cuda", devices=1, plugins=BitsandbytesPrecision("nf4-dq"))
+    model = Model()
+    model = fabric.setup(model)
+    state_dict = torch.load(tmp_path / "checkpoint.pt")
+    model.load_state_dict(state_dict)
+    assert model.linear.weight.dtype == torch.uint8
+    assert model.linear.weight.shape == (128, 1)
+    # Shapes match during forward (weight is being dequantized during forward)
+    model(torch.randn(2, 16, device=fabric.device))