chore: add documentation to support new onnx node (#599)

Co-authored-by: Roman Bredehoft <[email protected]>

docs/SUMMARY.md

@@ -63,6 +63,7 @@
 - [Documentation](developer/documenting.md)
 - [Support and issues](developer/debug_support_submit_issues.md)
 - [Contributing](developer/contributing.md)
+- [Support new ONNX node](developer/support_new_onnx_node.md)
 - [Release note](https://github.com/zama-ai/concrete-ml/releases)
 - [Feature request](https://github.com/zama-ai/concrete-ml/issues/new?assignees=&labels=feature&projects=&template=feature_request.md)
 - [Bug report](https://github.com/zama-ai/concrete-ml/issues/new?assignees=&labels=bug&projects=&template=bug_report.md)

docs/developer/support_new_onnx_node.md

@@ -0,0 +1,48 @@
+# Supporting New ONNX Nodes in Concrete ML
+
+Concrete ML supports a wide range of models through the integration of ONNX nodes. In case a specific ONNX node is missing, developers need to add support for the new ONNX nodes.
+
+## Operator Implementation
+
+### Floating-point Implementation
+
+The [`ops_impl.py`](../../src/concrete/ml/onnx/ops_impl.py) file is responsible for implementing the computation of ONNX operators using floating-point arithmetic. The implementation should mirror the behavior of the corresponding ONNX operator precisely. This includes adhering to the expected inputs, outputs, and operational semantics.
+
+Refer to the [ONNX documentation](https://github.com/onnx/onnx/blob/main/docs/Operators.md) to grasp the expected behavior, inputs and outputs of the operator.
+
+### Operator Mapping
+
+After implementing the operator in [`ops_impl.py`](../../src/concrete/ml/onnx/ops_impl.py), you need to import it into [`onnx_utils.py`](../../src/concrete/ml/onnx/onnx_utils.py) and map it within the `ONNX_OPS_TO_NUMPY_IMPL` dictionary. This mapping is crucial for the framework to recognize and utilize the new operator.
+
+### Quantized Operator
+
+Quantized operators are defined in [`quantized_ops.py`](../../src/concrete/ml/quantization/quantized_ops.py) and are used to handle integer arithmetic. Their implementation is required for the new ONNX to be executed in FHE.
+
+There exist two types of quantized operators:
+
+- **Univariate Non-Linear Operators**: Such operator applies transformation on every element of the input without changing its shape. Sigmoid, Tanh, ReLU are examples of such operation. The sigmoid in this file is simply supported as follows:
+
+<!--pytest-codeblocks:skip-->
+
+```python
+class QuantizedSigmoid(QuantizedOp):
+    """Quantized sigmoid op."""
+
+    _impl_for_op_named: str = "Sigmoid"
+```
+
+- **Linear Layers**: Linear layers like `Gemm` and `Conv` require specific implementations for integer arithmetic. Please refer to the `QuantizedGemm` and `QuantizedConv` implementations for reference.
+
+## Adding Tests
+
+Proper testing is essential to ensure the correctness of the new ONNX node support.
+
+There are many locations where tests can be added:
+
+- [`test_onnx_ops_impl.py`](../../tests/onnx/test_onnx_ops_impl.py): Tests the implementation of the ONNX node in floating points.
+- [`test_quantized_ops.py`](../../tests/quantization/test_quantized_ops.py): Tests the implementation of the ONNX node in integer arithmetic.
+- Optional: [`test_compile_torch.py`](../../tests/torch/test_compile_torch.py): Tests the implementation of a specific torch model that contains the new ONNX operator. The model needs to be added in [`torch_models.py`](../../src/concrete/ml/pytest/torch_models.py).
+
+## Update Documentation
+
+Finally, update the documentation to reflect the newly supported ONNX node.

docs/index.toc.txt

@@ -95,5 +95,6 @@
     developer/documenting.md
     developer/debug_support_submit_issues.md
     developer/contributing.md
+    developer/support_new_onnx_node.md