Typo in variable name: BETA --> self.BETA (#16666)

__BETA__ is defined on line 118 as a class member so it can only be accessed via __self__ or via the __ElasticAverageOptimizer__.

flake8 testing of https://github.com/tensorflow/tensorflow

$ __flake8 . --count --select=E901,E999,F821,F822,F823 --show-source --statistics__
```
./tensorflow/contrib/opt/python/training/elastic_average_optimizer.py:153:27: F821 undefined name 'BETA'
      self._moving_rate = BETA / communication_period / num_worker
                          ^
```