Why should I know C++ ,if I am a machine learning engineer?
Many machine learning products emphasize on speed over accuracy, e.g. in autonomous vehicles, a model that predicts in milliseconds with 98% accuracy is more valuable than a model that predicts in seconds with 99.99% accuracy. 

While Python provides many great packages for exploring machine learning research and testing proofs of concept, it is significantly slower than C++ and not acceptable for products with real-time requirements. If you want to know why Python is slower, check out this thread.