Draft: Export webnn via builder

Cargo.toml

@@ -13,7 +13,7 @@ crate-type = ["cdylib", "rlib"]
 name = "pywebnn"
 
 [features]
-default = ["onnx-runtime", "dynamic-inputs"]
+default = ["trtx-runtime", "onnx-runtime", "dynamic-inputs"]
 onnx-runtime = ["rustnn/onnx-runtime", "ort", "ndarray"]
 coreml-runtime = ["rustnn/coreml-runtime"]
 trtx-runtime = ["rustnn/trtx-runtime"]

docs/api-reference.md

@@ -1202,3 +1202,34 @@ Common exceptions:
 - `ValueError`: Invalid graph structure or parameters
 - `IOError`: File I/O errors
 - `RuntimeError`: Conversion or execution failures
+
+## RustNN builder text export
+
+Use a backend-free builder when you only need to construct and export a network:
+
+```python
+import webnn
+
+builder = webnn.MLGraphBuilder.new_uncompiled()
+x = builder.input("x", [2, 3], "float32")
+y = builder.relu(x)
+outputs = {"y": y}
+
+graph = builder.build(outputs)  # records an uncompiled MLGraph
+text = builder.rustnn_webnn_text_for_outputs(outputs)
+builder.rustnn_save_webnn(outputs, "model.webnn")
+```
+
+`rustnn_webnn_text_for_outputs(outputs) -> str` delegates to RustNN's text
+serializer and embeds constants inline. `rustnn_save_webnn(outputs, path)`
+delegates to RustNN's saver, writing the text file and a sibling
+`<stem>.safetensors` containing external constant weights. Both methods work
+before or after `build()` and leave the recorded graph unchanged. Empty,
+duplicate, invalid, or conflicting outputs raise an exception.
+
+`new_uncompiled()` initializes no backend and needs no runtime shared library.
+Its graph can later be executed by a context. Context-created builders expose
+the same serialization methods and retain their existing compilation behavior.
+The Python builder's recorded operations are replayed through
+`MLGraphBuilder::new_uncompiled()` for serialization, so RustNN owns shape
+validation, text formatting, and weight-file handling.

python/webnn/__init__.pyi

@@ -1,6 +1,7 @@
 """Type stubs for webnn package"""
 
 from typing import Any, Dict, List, Optional, Union
+import os
 import numpy as np
 import numpy.typing as npt
 
@@ -244,6 +245,19 @@ class MLGraphBuilder:
 
     def __init__(self) -> None: ...
 
+    @staticmethod
+    def new_uncompiled() -> MLGraphBuilder:
+        """Construct a builder without a runtime; build() returns an uncompiled graph."""
+        ...
+
+    def rustnn_webnn_text_for_outputs(self, outputs: Dict[str, MLOperand]) -> str:
+        """RustNN text serialization with inline constants, before or after build()."""
+        ...
+
+    def rustnn_save_webnn(self, outputs: Dict[str, MLOperand], path: Union[str, "os.PathLike[str]"]) -> None:
+        """Write RustNN text and <stem>.safetensors without consuming the builder."""
+        ...
+
     def input(
         self,
         name: str,

python/webnn/_rustnn.pyi

@@ -3,6 +3,7 @@
 from __future__ import annotations
 
 from typing import Dict, List, Optional
+from os import PathLike
 
 import numpy as np
 import numpy.typing as npt
@@ -74,6 +75,10 @@ class MLOperand:
 
 
 class MLGraphBuilder:
+    @staticmethod
+    def new_uncompiled() -> MLGraphBuilder: ...
+    def rustnn_webnn_text_for_outputs(self, outputs: Dict[str, MLOperand]) -> str: ...
+    def rustnn_save_webnn(self, outputs: Dict[str, MLOperand], path: str | PathLike[str]) -> None: ...
     def __init__(self) -> None: ...
 
     def build(self, outputs): ...