always use top_k call

[lzzzfelipe]

Fixes the error:

"Incompatible return types of true_fn and false_fn: {}".format(e))
TypeError: Incompatible return types of true_fn and false_fn: The two structures don't have the same sequence type. First structure has type <class 'tensorflow.python.ops.gen_nn_ops.TopKV2'>, while second structure has type <class 'list'>.

[RobertCsordas]

Calling top_k with less input elements than k results in an ugly runtime error, at least with older tensorflow versions, I don't know for new ones. That's why that cond is there. I made it compatible with new tensorflow, but have not removed it.