[webgpu] Simplify o2i_output implementation

[daijh]

Description

This change simplifies the o2i_output implementation by reducing unnecessary intermediate variables, with no change in functionality.

Motivation and Context

As above.

[daijh]

Before:

fn o2i_output(offset : u32)->vec3<u32> {
  var indices: vec3<u32>;
  var current = offset;
  let dim0 = current / uniforms.output_stride[0];
  let rest0 = current % uniforms.output_stride[0];
  indices[0] = dim0;
  current = rest0;
  let dim1 = current / uniforms.output_stride[1];
  let rest1 = current % uniforms.output_stride[1];
  indices[1] = dim1;
  current = rest1;
  indices[2] = current;
  return indices;
}

After:

fn o2i_output(offset : u32)->vec3<u32> {
  var indices: vec3<u32>;
  var current = offset;
  indices[0] = current / uniforms.output_stride[0];
  current = current % uniforms.output_stride[0];
  indices[1] = current / uniforms.output_stride[1];
  current = current % uniforms.output_stride[1];
  indices[2] = current;
  return indices;
}

[fs-eire]

/azp run Windows ARM64 QNN CI Pipeline,Windows x64 QNN CI Pipeline,Windows CPU CI Pipeline,Windows GPU CUDA CI Pipeline,Windows GPU DML CI Pipeline,Windows GPU Doc Gen CI Pipeline,Windows GPU TensorRT CI Pipeline,ONNX Runtime Web CI Pipeline,Linux CPU CI Pipeline,Linux CPU Minimal Build E2E CI Pipeline

[fs-eire]

/azp run Linux GPU CI Pipeline,Linux GPU TensorRT CI Pipeline,Linux OpenVINO CI Pipeline,Linux QNN CI Pipeline,MacOS CI Pipeline,orttraining-linux-ci-pipeline,orttraining-linux-gpu-ci-pipeline,onnxruntime-binary-size-checks-ci-pipeline,Big Models,Linux Android Emulator QNN CI Pipeline

[fs-eire]

/azp run Android CI Pipeline,iOS CI Pipeline,ONNX Runtime React Native CI Pipeline,CoreML CI Pipeline,Linux DNNL CI Pipeline,Linux MIGraphX CI Pipeline,Linux ROCm CI Pipeline

[azure-pipelines]

Azure Pipelines successfully started running 7 pipeline(s).

[azure-pipelines]

Azure Pipelines successfully started running 8 pipeline(s).

[fs-eire]

/azp run Windows GPU WebGPU CI Pipeline

[azure-pipelines]

Azure Pipelines successfully started running 10 pipeline(s).

[azure-pipelines]

No pipelines are associated with this pull request.

[fs-eire]

/azp run Win_TRT_Minimal_CUDA_Test_CI

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).