same calculation for CrossEntropy and NegativeLogLikelihood in mxnet.metric

[acphile]

CrossEntropy: http://mxnet.incubator.apache.org/api/python/docs/api/metric/index.html#mxnet.metric.CrossEntropy
NegativeLogLikelihood: http://mxnet.incubator.apache.org/api/python/docs/api/metric/index.html#mxnet.metric.NegativeLogLikelihood
Currently, in mxnet.metric, the definition of CrossEntropy is actually same as the definition of NegativeLogLikelihood, which leads to a duplicate in mxnet.metric.

[eric-haibin-lin]

i think we can remove one of them