Fix group attribute default at parse time instead of validation time

Co-authored-by: guschmue <22941064+guschmue@users.noreply.github.com>

Author: Copilot

js/web/lib/wasm/jsep/webgpu/ops/conv-transpose.ts

@@ -132,7 +132,7 @@ export const parseConvTransposeAttributes = (attributes: Record<string, unknown>
     typeof attributes.autoPad == 'undefined' ? 0 : (attributes.autoPad as number)
   ];
   const dilations = attributes.dilations as [number, number];
-  const group = attributes.group as number;
+  const group = (attributes.group as number) ?? 1;  // default to 1 per ONNX spec
   const kernelShape = attributes.kernelShape as [number, number];
   const pads = attributes.pads as [number, number, number, number];
   const strides = attributes.strides as [number, number];
@@ -183,8 +183,7 @@ const validateInputs = (inputs: readonly TensorView[], attributes: ConvTranspose
     if (inputs[2].dims.length !== 1) {
       throw new Error('invalid bias: bias must be 1D tensor');
     }
-    const group = attributes.group ?? 1;  // default to 1 if not specified (per ONNX spec)
-    const featureMaps = inputs[1].dims[1] * group;
+    const featureMaps = inputs[1].dims[1] * attributes.group;
     if (inputs[2].dims[0] !== featureMaps) {
       throw new Error(`invalid bias: bias size (${inputs[2].dims[0]}) must be equal to output channels (${featureMaps})`);
     }