Fixed code to use AdamWeightDecayOptimizer

[jinamshah]

Made changes to the code here.
This was done because when the code reaches the said line, the optimizer is not necessarily a AdamWeightDecayOptimizer object. It can be a CrossShardOptimizer object.
Thus, by change checking if the weight_decay flag is not equal to zero, we can conclusively say if the optimizer uses Adam weight decay or not.

Note: if FLAGS.weight_decay != 0 can be changed to if not FLAGS.weight_decay but that would hinder the code if someone has used None by mistake