PYSEC-2021-623

TensorFlow is an open source platform for machine learning. In affected versions the shape inference code for tf.ragged.cross has an undefined behavior due to binding a reference to nullptr. The fix will be included in TensorFlow 2.7.0. We will also cherrypick this commit on TensorFlow 2.6.1, TensorFlow 2.5.2, and TensorFlow 2.4.4, as these are also affected and still in supported range.