fix for webgl lrn (microsoft#15236)

fix issue that resulted in wrong results for lrn on webgpu

Author: guschmue

js/web/lib/onnxjs/backends/webgl/ops/lrn.ts

@@ -42,24 +42,14 @@ const lrnProgramMetadata = {
   inputTypes: [TextureType.unpacked]
 };
 
-function getOutputExpression(attributes: LrnAttributes): string {
-  let expression = `float(${attributes.bias}) + float(${attributes.alpha}) * square_sum`;
-  if (attributes.beta === 0.5) {
-    expression = `inversesqrt(${expression})`;
-  } else if (attributes.beta === 1.0) {
-    expression = `1.0/(${expression})`;
-  } else {
-    expression = `exp(log(${expression})) * float(-${attributes.beta})`;
-  }
-  return `x * ${expression}`;
-}
-
 function createLrnProgramInfo(inputs: Tensor[], attributes: LrnAttributes): ProgramInfo {
   const C = inputs[0].dims[1];
-
   const rank = inputs[0].dims.length;
   const from = -Math.floor((attributes.size - 1) / 2);
   const to = Math.ceil((attributes.size - 1) / 2);
+  const alpha = `float(${attributes.alpha}) / float(${attributes.size})`;
+  const bias = `float(${attributes.bias})`;
+  const beta = `float(${attributes.beta})`;
 
   const shaderSource = `
     float process(int indices[${rank}]) {
@@ -75,8 +65,7 @@ function createLrnProgramInfo(inputs: Tensor[], attributes: LrnAttributes): Prog
             square_sum += j * j;
           }
         }
-
-        return ${getOutputExpression(attributes)};
+        return x / pow(${bias} + ${alpha} * square_sum, ${beta});
     }`;
   return {
     ...lrnProgramMetadata,