message: Improve the error message when the user uses tf.while_loop() on TPUs.
diff: @@ -107,7 +107,9 @@ class StackOp : public XlaOpKernel {
  OP_REQUIRES(
  ctx, size >= 0,
  errors::InvalidArgument(
  - "XLA compilation requires a fixed stack size upper bound."));
  + "XLA compilation requires a fixed stack size upper bound. If "
  + "you are using tf.while_loop, set the maximum_iterations parameter "
  + "to fix this issue."));
  // We defer initializing the Stack resource until we see the first push.
  // Otherwise we do not know the shape of the stack elements.
  