Add last-layer Laplace flavors

laplace/curvature.py

@@ -6,12 +6,23 @@
 from backpack import backpack, extend
 from backpack.extensions import DiagGGNExact, DiagGGNMC, KFAC, KFLR, SumGradSquared, BatchGrad
 
-from laplace.jacobians import Jacobians
+from laplace.jacobians import Jacobians, LLJacobians
 from laplace.matrix import Kron
 
 
 class CurvatureInterface(ABC):
 
+    def __init__(self, model, likelihood):
+        assert likelihood in ['regression', 'classification']
+        self.likelihood = likelihood
+        self.model = model
+        if likelihood == 'regression':
+            self.lossfunc = MSELoss(reduction='sum')
+            self.factor = 0.5  # convert to standard Gauss. log N(y|f,1)
+        else:
+            self.lossfunc = CrossEntropyLoss(reduction='sum')
+            self.factor = 1.
+
     @abstractmethod
     def full(self, X, y, **kwargs):
         pass
@@ -24,19 +35,44 @@ def kron(self, X, y, **kwargs):
     def diag(self, X, y, **kwargs):
         pass
 
+    def _get_full_ggn(self, Js, f, y):
+        loss = self.factor * self.lossfunc(f, y)
+        if self.likelihood == 'regression':
+            H_ggn = torch.einsum('mkp,mkq->pq', Js, Js)
+        else:
+            # second derivative of log lik is diag(p) - pp^T
+            ps = torch.softmax(f, dim=-1)
+            H_lik = torch.diag_embed(ps) - torch.einsum('mk,mc->mck', ps, ps)
+            H_ggn = torch.einsum('mcp,mck,mkq->pq', Js, H_lik, Js)
+        return loss.detach(), H_ggn
+
+
+class LastLayer(CurvatureInterface):
+
+    def __init__(self, model, likelihood, backend, **kwargs):
+        super().__init__(model, likelihood)
+        self.backend = backend(self.model, self.likelihood, last_layer=True, **kwargs)
+
+    def diag(self, X, y, **kwargs):
+        return self.backend.diag(X, y, **kwargs)
+
+    def kron(self, X, y, **kwargs):
+        return self.backend.kron(X, y, **kwargs)
+
+    def full(self, X, y, **kwargs):
+        Js, f = LLJacobians(self.model, X)
+        loss, H = self._get_full_ggn(Js, f, y)
+
+        return loss, H
+
 
 class BackPackInterface(CurvatureInterface):
 
-    def __init__(self, model, likelihood):
-        assert likelihood in ['regression', 'classification']
-        self.likelihood = likelihood
-        self.model = extend(model)
-        if likelihood == 'regression':
-            self.lossfunc = extend(MSELoss(reduction='sum'))
-            self.factor = 0.5  # convert to standard Gauss. log N(y|f,1)
-        else:
-            self.lossfunc = extend(CrossEntropyLoss(reduction='sum'))
-            self.factor = 1.
+    def __init__(self, model, likelihood, last_layer=False):
+        super().__init__(model, likelihood)
+        self.last_layer = last_layer
+        extend(self.model.last_layer) if last_layer else extend(self.model)
+        extend(self.lossfunc)
 
 
 class BackPackGGN(BackPackInterface):
@@ -47,21 +83,29 @@ class BackPackGGN(BackPackInterface):
     --> factor for regression is 0.5 for loss and ggn
     """
 
-    def __init__(self, model, likelihood, stochastic=False):
-        super().__init__(model, likelihood)
+    def __init__(self, model, likelihood, last_layer=False, stochastic=False):
+        super().__init__(model, likelihood, last_layer)
         self.stochastic = stochastic
 
     def _get_diag_ggn(self):
+        if self.last_layer:
+            model = self.model.last_layer
+        else:
+            model = self.model
         if self.stochastic:
-            return torch.cat([p.diag_ggn_mc.data.flatten() for p in self.model.parameters()])
+            return torch.cat([p.diag_ggn_mc.data.flatten() for p in model.parameters()])
         else:
-            return torch.cat([p.diag_ggn_exact.data.flatten() for p in self.model.parameters()])
+            return torch.cat([p.diag_ggn_exact.data.flatten() for p in model.parameters()])
 
     def _get_kron_factors(self):
+        if self.last_layer:
+            model = self.model.last_layer
+        else:
+            model = self.model
         if self.stochastic:
-            return Kron([p.kfac for p in self.model.parameters()])
+            return Kron([p.kfac for p in model.parameters()])
         else:
-            return Kron([p.kflr for p in self.model.parameters()])
+            return Kron([p.kflr for p in model.parameters()])
 
     @staticmethod
     def _rescale_kron_factors(kron, M, N):
@@ -98,15 +142,9 @@ def full(self, X, y, **kwargs):
             raise ValueError('Stochastic approximation not implemented for full GGN.')
 
         Js, f = Jacobians(self.model, X)
-        loss = self.factor * self.lossfunc(f, y)
-        if self.likelihood == 'regression':
-            H_ggn = torch.einsum('mkp,mkq->pq', Js, Js)
-        else:
-            # second derivative of log lik is diag(p) - pp^T
-            ps = torch.softmax(f, dim=-1)
-            H_lik = torch.diag_embed(ps) - torch.einsum('mk,mc->mck', ps, ps)
-            H_ggn = torch.einsum('mcp,mck,mkq->pq', Js, H_lik, Js)
-        return loss.detach(), H_ggn
+        loss, H_ggn = self._get_full_ggn(Js, f, y)
+
+        return loss, H_ggn
 
 
 class BackPackEF(BackPackInterface):

laplace/feature_extractor.py

@@ -0,0 +1,123 @@
+import torch
+import torch.nn as nn
+from typing import Tuple, Callable, Optional
+
+
+class FeatureExtractor(nn.Module):
+    """Feature extractor for a PyTorch neural network.
+    A wrapper which returns the output of the penultimate layer in addition to
+    the output of the last layer for each forward pass. It assumes that the
+    last layer is linear.
+    Based on https://gist.github.com/fkodom/27ed045c9051a39102e8bcf4ce31df76.
+
+    Arguments
+    ----------
+    model : torch.nn.Module
+        PyTorch model
+
+    last_layer_name (optional) : str, default=None
+        If the user already knows the name of the last layer, otherwise it will
+        be determined automatically.
+
+    Attributes
+    ----------
+    model : torch.nn.Module
+        The underlying PyTorch model.
+
+    last_layer : torch.nn.module
+        The torch module corresponding to the last layer (has to be instance
+        of torch.nn.Linear).
+
+    Examples
+    --------
+
+    Notes
+    -----
+    Limitations:
+        - Assumes that the last layer is always the same for any forward pass
+        - Assumes that the last layer is an instance of torch.nn.Linear
+    """
+    def __init__(self, model: nn.Module, last_layer_name: Optional[str] = None) -> None:
+        super().__init__()
+        self.model = model
+        self._features = dict()
+        if last_layer_name is None:
+            self._found = False
+        else:
+            self.set_last_layer(last_layer_name)
+
+    def forward(self, x: torch.Tensor) -> torch.Tensor:
+        if self._found:
+            # if last and penultimate layers are already known
+            out = self.model(x)
+        else:
+            # if this is the first forward pass
+            out = self.find_last_layer(x)
+        return out
+
+    def forward_with_features(self, x: torch.Tensor) -> Tuple[torch.Tensor, torch.Tensor]:
+        out = self.forward(x)
+        features = self._features[self._last_layer_name]
+        return out, features
+
+    def set_last_layer(self, last_layer_name: str) -> None:
+        # set last_layer attributes and check if it is linear
+        self._last_layer_name = last_layer_name
+        self.last_layer = dict(self.model.named_modules())[last_layer_name]
+        if not isinstance(self.last_layer, nn.Linear):
+            raise ValueError('Use model with a linear last layer.')
+
+        # set forward hook to extract features in future forward passes
+        self.last_layer.register_forward_hook(self._get_hook(last_layer_name))
+
+        # last layer is now identified and hook is set
+        self._found = True
+
+    def _get_hook(self, name: str) -> Callable:
+        def hook(_, input, __):
+            # only accepts one input (expects linear layer)
+            self._features[name] = input[0].detach()
+        return hook
+
+    def find_last_layer(self, x: torch.Tensor) -> torch.Tensor:
+        if self._found:
+            raise ValueError('Last layer is already known.')
+
+        act_out = dict()
+        def get_act_hook(name):
+            def act_hook(_, input, __):
+                # only accepts one input (expects linear layer)
+                if isinstance(input[0], torch.Tensor):
+                    act_out[name] = input[0].detach()
+                else:
+                    act_out[name] = None
+                # remove hook
+                handles[name].remove()
+            return act_hook
+
+        # set hooks for all modules
+        handles = dict()
+        for name, module in self.model.named_modules():
+            handles[name] = module.register_forward_hook(get_act_hook(name))
+
+        # check if model has more than one module
+        # (there might be pathological exceptions)
+        if len(handles) <= 2:
+            raise ValueError('The model only has one module.')
+
+        # forward pass to find execution order
+        out = self.model(x)
+
+        # find the last layer, store features, return output of forward pass
+        keys = list(act_out.keys())
+        for key in reversed(keys):
+            layer = dict(self.model.named_modules())[key]
+            if len(list(layer.children())) == 0:
+                self.set_last_layer(key)
+
+                # save features from first forward pass
+                self._features[key] = act_out[key]
+
+                return out
+
+        raise ValueError('Something went wrong (all modules have children).')

laplace/jacobians.py

@@ -40,3 +40,18 @@ def Jacobians(model, data):
         return torch.stack(to_stack, dim=2).transpose(1, 2), f
     else:
         return Jk.unsqueeze(-1).transpose(1, 2), f
+
+
+def LLJacobians(model, data):
+    f, phi = model.forward_with_features(data)
+    bsize = len(data)
+    output_size = f.shape[-1]
+
+    # calculate Jacobians using the feature vector 'phi'
+    identity = torch.eye(output_size, device=data.device).unsqueeze(0).tile(bsize, 1, 1)
+    # Jacobians are batch x output x params
+    Js = torch.einsum('kp,kij->kijp', phi, identity).reshape(bsize, output_size, -1)
+    if model.last_layer.bias is not None:
+        Js = torch.cat([Js, identity], dim=2)
+
+    return Js, f