`CHECK`-fail in `MapStage`
ImpactAn attacker can trigger a denial of service via a `CHECK`-fail in `tf.raw_ops.MapStage`:The [implementation](https://github.com/tensorflow/tensorflow/blob/460e000de3a83278fb00b61a16d161b1964f15f4/tensorflow/core/kernels/map_stage_op.cc#L513) does not check that the `key` input is a valid non-empty tensor.PatchesWe have patched the issue in GitHub commit [d7de67733925de196ec8863a33445b73f9562d1d](https://github.com/tensorflow/tensorflow/commit/d7de67733925de196ec8863a33445b73f9562d1d).The fix will be included in TensorFlow 2.6.0. We will also cherrypick this commit on TensorFlow 2.5.1, TensorFlow 2.4.3, and TensorFlow 2.3.4, as these are also affected and still in supported range.For more informationPlease consult [our security  guide](https://github.com/tensorflow/tensorflow/blob/master/SECURITY.md) for more information regarding the security model and how to contact us with issues and questions.AttributionThis vulnerability has been reported by Ying Wang and Yakun Zhang of Baidu X-Team.