Reword "models are data structures ..." (#153)
Reword "models are data structures ..." (#153)

* Reword "models are data structures ..."

@lattner pointed out that this could be made more clear. Updated to

> Machine learning models are concepturally functions with internal state called "parameters". In code, models are often represented as a data structure that stores parameters as mutable properties and has an "apply" method.

* Update docs/ParameterOptimization.md

Co-Authored-By: rxwei <3a39a3ee6ebc99bd2c549d35b83062bd13ec047a@google.com>

tensorflow/swift,tensorflow/swift