Fix inconsistent MLTensor usage in Code example (#291)

Fixed #290

Author: Honry

code/samples/mul_add.js

@@ -22,6 +22,7 @@ const tensorC = await context.createTensor({
   ...desc,
   usage: MLTensorUsage.READ,
   readable: true,
+  writable: false,
 });
 const inputs = {'A': tensorA, 'B': tensorB};
 const outputs = {'C': tensorC};

code/samples/simple_graph.js

@@ -62,6 +62,7 @@ const outputTensor = await context.createTensor({
   ...desc,
   usage: MLTensorUsage.READ,
   readable: true,
+  writable: false,
 });
 
 // Execute the compiled graph with the specified inputs.