exatn::sync(TensorOperation) is inconsistent

[DmitryLyakh]

exatn::sync executed on exatn::TensorOperation uses its Id assigned during the operation submission. This Id refers to a local DAG node. Two potential problems: (1) Attempt to sync a tensor operation that has never been submitted will result in a SegFault since the Id is not set yet; (2) The global exatn::sync is expected to destroy the current DAG, so a later attempt to access its node will result in a SegFault. A proper way to sync a tensor operation is to sync every output operand.