PYSEC-2021-809

TensorFlow is an open source platform for machine learning. In affeced versions during execution, EinsumHelper::ParseEquation() is supposed to set the flags in input_has_ellipsis vector and *output_has_ellipsis boolean to indicate whether there is ellipsis in the corresponding inputs and output. However, the code only changes these flags to true and never assigns false. This results in unitialized variable access if callers assume that EinsumHelper::ParseEquation() always sets these flags. 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.