message: [tf.data] Fixing a bug in TFRecordWriter.
  The problem was that the op kernel was not originally creating the `ResourceMgr` parameter of `IteratorContext`, which would cause any upstream dataset op that creates resources (such as `shuffle` or `cache`) to segfault.
diff: @@ -15,6 +15,7 @@ limitations under the License.
  #include "tensorflow/core/framework/dataset.h"
  #include "tensorflow/core/framework/function_handle_cache.h"
  #include "tensorflow/core/framework/op_kernel.h"
  +#include "tensorflow/core/framework/resource_mgr.h"
  #include "tensorflow/core/kernels/data/dataset_utils.h"
  #include "tensorflow/core/kernels/ops_util.h"
  #include "tensorflow/core/lib/core/threadpool.h"
  @@ -71,6 +72,8 @@ class ToTFRecordOp : public AsyncOpKernel {
  std::unique_ptr<FunctionHandleCache> function_handle_cache =
  absl::make_unique<FunctionHandleCache>(params.flr);
  params.function_handle_cache = function_handle_cache.get();
  + auto resource_mgr = absl::make_unique<ResourceMgr>();
  + params.resource_mgr = resource_mgr.get();
  IteratorContext iter_ctx(std::move(params));
  OP_REQUIRES_OK_ASYNC(
  