fix(session): suppress VerifyEachNodeIsAssignedToAnEp warning for explicit EP

src/winml/modelkit/session/session.py

@@ -423,11 +423,9 @@ def _build_session_options(self, device: str) -> ort.SessionOptions:
 
         When ``self._ep`` is set (and not ``"cpu"``), uses
         ``add_provider_for_devices`` to explicitly bind that EP.
-        ``"cpu"`` falls through to policy-based selection so ORT handles
-        CPU-only inference without any EP registration.
-        When ``self._ep`` is not set, queries ``get_ep_devices()`` to
-        discover an available EP for the target device type. Falls back to
-        policy-based selection only as a last resort.
+        Otherwise uses policy-based selection via DEVICE_POLICY_MAP so ORT
+        handles provider assignment (including CPU fallback for unsupported
+        ops) without triggering ``VerifyEachNodeIsAssignedToAnEp`` warnings.
 
         Note: Returns a **fresh** SessionOptions when using explicit EP to
         avoid "already registered" errors from repeated calls.
@@ -445,6 +443,12 @@ def _build_session_options(self, device: str) -> ort.SessionOptions:
 
                     opts = ort.SessionOptions()
                     opts.add_provider_for_devices([matched], self._provider_options)
+                    # Register CPU as explicit fallback so shape-related ops
+                    # have a registered home and VerifyEachNodeIsAssignedToAnEp
+                    # does not warn about unassigned nodes.
+                    cpu_dev = self._find_ep_device(ep_name="CPUExecutionProvider", device="cpu")
+                    if cpu_dev:
+                        opts.add_provider_for_devices([cpu_dev], {})
                     resolved = DEVICE_TYPE_TO_DEVICE.get(
                         matched.device.type, str(matched.device.type)
                     )
@@ -463,16 +467,7 @@ def _build_session_options(self, device: str) -> ort.SessionOptions:
                     device,
                 )
 
-        # No explicit EP — discover available EP for this device type
-        if not self._ep and device.lower() != "cpu":
-            matched = self._find_ep_device(device=device)
-            if matched:
-                opts = ort.SessionOptions()
-                opts.add_provider_for_devices([matched], self._provider_options)
-                logger.info("Discovered EP for %s: %s", device, matched.ep_name)
-                return opts
-
-        # Policy-based selection (last resort)
+        # Policy-based selection (default when no explicit EP)
         opts = self._session_options
         policy = DEVICE_POLICY_MAP.get(
             device.lower(), ort.OrtExecutionProviderDevicePolicy.PREFER_NPU
@@ -486,23 +481,13 @@ def _build_session_options(self, device: str) -> ort.SessionOptions:
     def _find_ep_device(device: str, ep_name: str | None = None) -> Any:
         """Find the first OrtEpDevice matching the given filters.
 
-        Behavior:
-            - ``ep_name`` set, ``device == "auto"`` → first ep_device
-              matching ``ep_name`` (or None).
-            - ``ep_name`` unset, ``device == "auto"`` → ``None`` (no
-              effective filter — refuse to pick an arbitrary ep_device).
-            - ``ep_name`` unset, ``device`` is a concrete type → first
-              ep_device matching that device type (or None).
-            - Both set → ep_device must satisfy both (or None).
-
-        Note: Selection order is determined by the ORT EP registry, which is
-        not part of any documented contract. On systems where multiple EPs
-        match the same device type (e.g., QNN and DML both appear as GPU),
-        a device-only query returns the first one in registry order. Pass
-        ``ep_name`` to disambiguate.
+        Filters are AND'd: both ``ep_name`` and ``device`` (when concrete)
+        must match. ``device="auto"`` acts as no-op device filter.
+        Returns ``None`` when no effective filter is provided (i.e. both
+        ``ep_name`` is None and ``device`` resolves to no device type).
 
         Args:
-            device: Device policy ("cpu", "gpu", "npu", "auto"). ``"auto"``
+            device: Device type ("cpu", "gpu", "npu", "auto"). ``"auto"``
                 and unknown strings act as no-op device filters.
             ep_name: Full EP name (e.g., "DmlExecutionProvider"), or None
                 to skip EP-name filtering.

tests/unit/session/test_winml_session.py

@@ -719,19 +719,6 @@ def test_ep_name_only(self) -> None:
         assert match is not None
         assert match.ep_name == "QNNExecutionProvider"
 
-    def test_device_only(self) -> None:
-        """device filter alone returns first matching device type."""
-        import onnxruntime as ort
-
-        devs = [
-            self._ep_dev("CPUExecutionProvider", ort.OrtHardwareDeviceType.CPU),
-            self._ep_dev("DmlExecutionProvider", ort.OrtHardwareDeviceType.GPU),
-        ]
-        with self._patch_devices(devs):
-            match = WinMLSession._find_ep_device(device="gpu")
-        assert match is not None
-        assert match.ep_name == "DmlExecutionProvider"
-
     def test_ep_name_and_device_both_required(self) -> None:
         """When both filters are set, both must match (AND logic)."""
         import onnxruntime as ort