Fixed AdamWeightDecayOptimizer usage code

[jinamshah]

@zihangdai @kimiyoung
I have raised a PR for this:
#146

What is affected by this issue:
Because of this piece of code, the model will kep training on step 0 if AdamWeightDecayOptimizer is used with TPU
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

Please let me know if any other changes are required