message: Add comment about use of IsIdentityAfterVariable
diff: @@ -1441,6 +1441,9 @@ void AutoMixedPrecisionImpl::PropagateWhiteThroughClear(
  ShouldProcess(*item.node) && IsFloat32(item) &&
  SupportsFloat16(item) &&
  (fp16_clearlist_.count(item.node->op())) &&
  + // We don't propagate (backwards) through Identity nodes when
  + // they immediately follow Variable nodes because otherwise it
  + // breaks TensorBoard visualization.
  !IsIdentityAfterVariable(*item.node));
  }),
  DfsTypeCallbacks::PreOrder([&](int idx) {
  