[WebGPU EP] fixes bugs in split implementation (#24259)
Fixes the following errors:
```
[ONNXRuntimeError] : 1 : FAIL : WebGPU validation failed. Error while parsing WGSL: :48:1 error: unexpected token } ^ - While validating [ShaderModuleDescriptor] - While calling [Device].CreateShaderModule([ShaderModuleDescriptor]).
```
```
[E:onnxruntime:sam, sequential_executor.cc:572 onnxruntime::ExecuteKernel] Non-zero status code returned while running Split node. Name:'/Split_1' Status Message: WebGPU validation failed. Error while parsing WGSL: :62:14 error: cannot index type 'u32' index -= uniforms.sizes_in_split_axis[output_number - 1u];
```
