[ModelSuite] Add Toy Models

Summary:

Here we introduce model suite (model.py). The idea here to start and codify the ideas from jiannanWang/BackendBenchExamples.  Specifically this PR adds some example models / configs which are to be loaded + a Readme. (It may be useful to look at the PR above this as well since it's the model loading logic).

This PR adds two toy models to model suite

SmokeTestModel - This is simple model that uses aten.ops.mm as we can implement a correct version of this op
ToyCoreOpsModel - This is a model which explicitly calls the backwards passes which are both in torchbench + core.

Test Plan:
the test infra is in the pr above, so tests passing on the PR above should be sufficient here

### Future work with Model Suite
#181

Author: PaliC

BackendBench/suite/models/README.md

@@ -0,0 +1,80 @@
+# Adding Models to BackendBench
+
+## Quick Start
+
+Models define operator lists and validate that custom backends work correctly in full model execution. Two files required:
+
+```
+BackendBench/suite/models/YourModel/
+├── YourModel.py      # nn.Module class
+└── YourModel.json    # Configuration
+```
+
+**Naming rule:** Directory name = File name = Class name (exact match, case-sensitive)
+
+## Adding a Model
+
+### 1. Create Directory and Files
+
+```bash
+cd BackendBench/suite/models
+mkdir MyModel
+cd MyModel
+touch MyModel.py MyModel.json
+```
+
+### 2. Write Model Class (`MyModel.py`)
+
+**Requirements:**
+- Class name = filename (exact match)
+- All `__init__` params need defaults
+- Add a main() / runner if you are inclined for sanity checking
+- Keep it simple - focus on specific operators you're testing
+- Look in this directory for examples
+
+### 3. Write Config (`MyModel.json`)
+
+**Key Fields:**
+- `model_config.init_args` - Args for `__init__()`, must match your defaults
+- `ops.forward` / `ops.backward` - Aten operators to test (format: `"aten.<op>.default"`)
+- `model_tests` - Test inputs as `"([], {kwarg: T([shape], dtype)})"` The format is further described [here](https://huggingface.co/datasets/GPUMODE/backendbench_tests#serialized-arguments-in-backendbench)
+  - Supported dtypes: `f32`, `f64`, `i32`, `i64`, `bool`, etc.
+- `metadata.description` - What this model tests
+- Look in this directory for examples
+
+**Finding operator names:**
+```python
+from torch.profiler import profile, ProfilerActivity
+
+with profile(activities=[ProfilerActivity.CPU]) as prof:
+    output = model(x)
+    loss = output.sum()
+    loss.backward()
+
+for event in prof.key_averages():
+    if "aten::" in event.key:
+        print(event.key)
+```
+
+### 4. Test Your Model
+
+```bash
+# Test standalone
+cd BackendBench/suite/models/MyModel
+python MyModel.py  # Add main() for standalone testing
+
+# Test with suite
+python -m BackendBench.scripts.main \
+    --suite model \
+    --backend aten \
+    --model-filter MyModel
+
+# Expected output:
+# Model: MyModel
+# Status: ✓ Passed (2/2 tests)
+#   ✓ small
+#   ✓ large
+```
+
+### 5: Validation
+`test/test_model_ops_configs.py` and `test/test_model_ops_coverage.py` are tests that validate that all models are loadable / formatted correctly.

BackendBench/suite/models/SmokeTestModel/SmokeTestModel.json

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+{
+  "model_config": {
+    "init_args": {
+      "input_dim": 128,
+      "hidden_dim": 128,
+      "output_dim": 128
+    }
+  },
+  "ops": {
+    "forward": [
+      "aten.mm.default"
+    ],
+    "backward": [
+      "aten.mm.default"
+    ]
+  },
+  "model_tests": {
+    "small_batch": "([], {'x': T([2, 128], f32)})",
+    "medium_batch": "([], {'x': T([16, 128], f32)})",
+    "large_batch": "([], {'x': T([32, 128], f32)})"
+  },
+  "metadata": {
+    "description": "Smoke test model focused on matrix multiplication operations (mm) in forward and backward passes"
+  }
+}

BackendBench/suite/models/SmokeTestModel/SmokeTestModel.py

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+# Copyright (c) Meta Platforms, Inc. and affiliates.
+# All rights reserved.
+#
+# This source code is licensed under the BSD 3-Clause license found in the
+# LICENSE file in the root directory of this source tree.
+
+"""
+Simple model that tests matrix multiplication operations using explicit
+torch.mm calls.
+"""
+
+import torch
+import torch.nn as nn
+
+
+class SmokeTestModel(nn.Module):
+    """
+    Model that uses explicit torch.mm operations to test aten.mm.default
+    in forward/backward.
+    """
+
+    def __init__(
+        self,
+        input_dim: int = 128,
+        hidden_dim: int = 128,
+        output_dim: int = 128,
+    ):
+        super().__init__()
+        self.input_dim = input_dim
+        self.hidden_dim = hidden_dim
+        self.output_dim = output_dim
+
+        self.weight1 = nn.Parameter(torch.randn(input_dim, hidden_dim))
+        self.weight2 = nn.Parameter(torch.randn(hidden_dim, output_dim))
+        self.bias1 = nn.Parameter(torch.randn(hidden_dim))
+        self.bias2 = nn.Parameter(torch.randn(output_dim))
+
+    def forward(self, x: torch.Tensor) -> torch.Tensor:
+        """
+        Forward pass: (x @ weight1 + bias1) -> relu -> (x @ weight2 + bias2)
+        """
+        x = torch.mm(x, self.weight1) + self.bias1
+        x = torch.relu(x)
+        x = torch.mm(x, self.weight2) + self.bias2
+        return x
+
+
+def main():
+    """Demonstrate the model with a forward/backward pass."""
+    model = SmokeTestModel(input_dim=128, hidden_dim=128, output_dim=128)
+    batch_size = 4
+    input_tensor = torch.randn(batch_size, 128, requires_grad=True)
+
+    model.train()
+    output = model(input_tensor)
+    loss = output.sum()
+    loss.backward()
+
+    print("✓ Forward/backward pass completed")
+    print(f"  Parameters: {sum(p.numel() for p in model.parameters())}")
+    print(f"  Input: {input_tensor.shape} -> Output: {output.shape}")
+    grad_count = sum(1 for p in model.parameters() if p.grad is not None)
+    total_params = len(list(model.parameters()))
+    print(f"  Gradients computed: {grad_count}/{total_params}")
+
+
+if __name__ == "__main__":
+    main()

BackendBench/suite/models/ToyCoreOpsModel/ToyCoreOpsModel.json

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+{
+  "model_config": {
+    "init_args": {
+      "in_channels": 3,
+      "hidden_channels": 32,
+      "out_channels": 8,
+      "num_groups": 8
+    }
+  },
+  "ops": {
+    "forward": [
+      "aten.convolution.default",
+      "aten.native_group_norm.default",
+      "aten.max_pool2d_with_indices.default",
+      "aten.avg_pool2d.default",
+      "aten._adaptive_avg_pool2d.default"
+    ],
+    "backward": [
+      "aten.convolution_backward.default",
+      "aten.native_group_norm_backward.default",
+      "aten.max_pool2d_with_indices_backward.default",
+      "aten.avg_pool2d_backward.default",
+      "aten._adaptive_avg_pool2d_backward.default"
+    ]
+  },
+  "model_tests": {
+    "small_batch": "([], {'x': T([2, 3, 32, 32], f32)})",
+    "medium_batch": "([], {'x': T([4, 3, 64, 64], f32)})",
+    "large_input": "([], {'x': T([2, 3, 128, 128], f32)})"
+  },
+  "metadata": {
+    "description": "Core operations model testing fundamental operators: convolution, group norm, max pool with indices, avg pool, adaptive avg pool"
+  }
+}

BackendBench/suite/models/ToyCoreOpsModel/ToyCoreOpsModel.py

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+# Copyright (c) Meta Platforms, Inc. and affiliates.
+# All rights reserved.
+#
+# This source code is licensed under the BSD 3-Clause license found in the
+# LICENSE file in the root directory of this source tree.
+
+"""
+CNN model that triggers core PyTorch backward operators:
+- convolution_backward
+- native_group_norm_backward
+- max_pool2d_with_indices_backward
+- avg_pool2d_backward
+- _adaptive_avg_pool2d_backward
+"""
+
+import torch
+import torch.nn as nn
+import torch.nn.functional as F
+
+
+class ToyCoreOpsModel(nn.Module):
+    """CNN that uses conv, group norm, max pool, avg pool, and adaptive avg pool."""
+
+    def __init__(
+        self,
+        in_channels: int = 3,
+        hidden_channels: int = 32,
+        out_channels: int = 8,
+        num_groups: int = 8,
+    ):
+        super().__init__()
+
+        if hidden_channels % num_groups != 0:
+            raise ValueError(
+                f"hidden_channels ({hidden_channels}) must be divisible by "
+                f"num_groups ({num_groups})"
+            )
+
+        self.in_channels = in_channels
+        self.hidden_channels = hidden_channels
+        self.out_channels = out_channels
+        self.num_groups = num_groups
+
+        self.conv1 = nn.Conv2d(in_channels, hidden_channels, kernel_size=3, padding=1)
+        self.group_norm1 = nn.GroupNorm(num_groups, hidden_channels)
+        self.conv2 = nn.Conv2d(hidden_channels, hidden_channels, kernel_size=3, padding=1)
+        self.group_norm2 = nn.GroupNorm(num_groups, hidden_channels)
+        self.conv_out = nn.Conv2d(hidden_channels, out_channels, kernel_size=1)
+
+    def forward(self, x: torch.Tensor) -> torch.Tensor:
+        """
+        Forward pass through: Conv->GroupNorm->ReLU->MaxPool->Conv->
+        GroupNorm->ReLU->AvgPool->AdaptiveAvgPool->Conv
+        Output is always (batch, out_channels, 4, 4) regardless of
+        input size.
+        """
+        x = F.relu(self.group_norm1(self.conv1(x)))
+        x, _ = F.max_pool2d(x, kernel_size=2, return_indices=True)
+        x = F.relu(self.group_norm2(self.conv2(x)))
+        x = F.avg_pool2d(x, kernel_size=2)
+        x = F.adaptive_avg_pool2d(x, output_size=(4, 4))
+        x = self.conv_out(x)
+        return x
+
+
+def main():
+    """Demonstrate the model with a forward/backward pass."""
+    model = ToyCoreOpsModel(in_channels=3, hidden_channels=32, out_channels=8, num_groups=8)
+    batch_size = 2
+    input_tensor = torch.randn(batch_size, 3, 64, 64, requires_grad=True)
+
+    model.train()
+    output = model(input_tensor)
+    loss = output.sum()
+    loss.backward()
+
+    print("✓ Forward/backward pass completed")
+    print(f"  Parameters: {sum(p.numel() for p in model.parameters())}")
+    print(f"  Input: {input_tensor.shape} -> Output: {output.shape}")
+    grad_count = sum(1 for p in model.parameters() if p.grad is not None)
+    total_params = len(list(model.parameters()))
+    print(f"  Gradients computed: {grad_count}/{total_params}")
+    return model
+
+
+if __name__ == "__main__":
+    main()