Use unpacked div always while we figure out why packed div produces NaNs.

BUG

Author: annxingyuan

src/kernels/backend_webgl.ts

@@ -1341,9 +1341,11 @@ export class MathBackendWebGL implements KernelBackend {
   realDivide(a: Tensor, b: Tensor): Tensor {
     const op = binaryop_gpu.DIV;
     const outputDtype = 'float32';
-    if (ENV.get('WEBGL_PACK_BINARY_OPERATIONS')) {
-      return this.packedBinaryOp(a, b, binaryop_packed_gpu.DIV, outputDtype);
-    }
+    // TODO: https://github.com/tensorflow/tfjs/issues/1324
+    // Revive this once we understand why this produces NaNs.
+    // if (ENV.get('WEBGL_PACK_BINARY_OPERATIONS')) {
+    //   return this.packedBinaryOp(a, b, binaryop_packed_gpu.DIV, outputDtype);
+    // }
     const program = new BinaryOpProgram(op, a.shape, b.shape);
     const output = this.makeOutputArray(program.outputShape, outputDtype);
     return this.compileAndRun<Tensor>(program, [a, b], output);