*   Migrate linear optimizer from contrib to core.

## Thanks to our Contributors

This release contains contributions from many people at Google, as well as:

Abhinav Upadhyay, Ag Ramesh, akikaaa, Alexis Louis, Anders Huss, Andreas Madsen,
Andrew Banchich, Andy Craze, Anton Dmitriev, Artem Malykh, Avijit-Nervana,
Balint Cristian, Benjamin Tan Wei Hao, Bhavani Subramanian, Brendan Finan, Brian