message: Clean up in sparse_fill_empty_rows_op.cc
  Both `int64 prev_row = -1;` and `const int64 row = indices(i, 0);`
  was not used. It generates quite a few warnings in the output.
diff: @@ -114,13 +114,11 @@ class SparseFillEmptyRowsOp : public OpKernel {
  &scratch_t));
  auto scratch = scratch_t.vec<int64>();
  scratch.device(d) = scratch.constant(0);
  - int64 prev_row = -1;
  for (int i = 0; i < N; ++i) {
  const int64 row = indices(i, 0);
  - OP_REQUIRES(context, indices(i, 0) >= 0 && indices(i, 0) < dense_rows,
  + OP_REQUIRES(context, row >= 0 && row < dense_rows,
  errors::InvalidArgument("indices(", i, ", 0) is invalid: ",
  - indices(i, 0), " >= ", dense_rows));
  - prev_row = row;
  + row, " >= ", dense_rows));
  ++scratch(indices(i, 0));
  }
  for (int row = 0; row < dense_rows; ++row) {
  