Initial implementation: all models, methods, toy and CIFAR experiments

Debug phase. Toy LQ experiments (3 seeds) complete with terminal gradient matching.
Credit bridge matches state bridge on linear system (~0.94 cosine).
CIFAR experiments in progress.

1 files changed, 73 insertions, 0 deletions

diff --git a/models/residual_mlp.py b/models/residual_mlp.py
new file mode 100644
index 0000000..c16778c
--- /dev/null
+++ b/models/residual_mlp.py
@@ -0,0 +1,73 @@
+"""
+Deep Residual MLP for classification.
+Architecture: Input -> Linear embedding -> L residual blocks -> LayerNorm -> Linear output head.
+Each block: h_{l+1} = h_l + W2 * GELU(W1 * LN(h_l))
+"""
+import torch
+import torch.nn as nn
+
+
+class ResidualBlock(nn.Module):
+    """Single pre-LayerNorm residual MLP block."""
+
+    def __init__(self, d_hidden: int):
+        super().__init__()
+        self.ln = nn.LayerNorm(d_hidden)
+        self.w1 = nn.Linear(d_hidden, d_hidden)
+        self.w2 = nn.Linear(d_hidden, d_hidden)
+        # Small init for residual branch
+        nn.init.normal_(self.w2.weight, std=0.01)
+        nn.init.zeros_(self.w2.bias)
+
+    def forward(self, h: torch.Tensor) -> torch.Tensor:
+        """Returns the residual F_l(h), NOT h + F_l(h)."""
+        z = self.ln(h)
+        z = self.w1(z)
+        z = torch.nn.functional.gelu(z)
+        z = self.w2(z)
+        return z
+
+
+class ResidualMLP(nn.Module):
+    """Deep residual MLP: embed -> L blocks -> LN -> output head."""
+
+    def __init__(self, input_dim: int, d_hidden: int, num_classes: int, num_blocks: int):
+        super().__init__()
+        self.embed = nn.Linear(input_dim, d_hidden)
+        self.blocks = nn.ModuleList([ResidualBlock(d_hidden) for _ in range(num_blocks)])
+        self.out_ln = nn.LayerNorm(d_hidden)
+        self.out_head = nn.Linear(d_hidden, num_classes)
+        self.num_blocks = num_blocks
+        self.d_hidden = d_hidden
+
+    def forward(self, x: torch.Tensor, return_hidden: bool = False):
+        """
+        Args:
+            x: (batch, input_dim)
+            return_hidden: if True, also return list of hidden states [h_0, ..., h_L]
+        Returns:
+            logits: (batch, num_classes)
+            hiddens: list of (batch, d_hidden) if return_hidden
+        """
+        h = self.embed(x)
+        hiddens = [h] if return_hidden else None
+
+        for block in self.blocks:
+            f = block(h)
+            h = h + f
+            if return_hidden:
+                hiddens.append(h)
+
+        logits = self.out_head(self.out_ln(h))
+
+        if return_hidden:
+            return logits, hiddens
+        return logits
+
+    def forward_from_layer(self, h: torch.Tensor, start_layer: int):
+        """Run forward from a given layer index to output. Used for perturbation tests."""
+        for i in range(start_layer, self.num_blocks):
+            f = self.blocks[i](h)
+            h = h + f
+        logits = self.out_head(self.out_ln(h))
+        return logits