* fix bug of using ignore_index and reduction,test=develop

* fix bug of celoss when using ignore_index and reduction, test=develop

* improve performance when ignore_index=-100, test=develop

* add test in test_cross_entropy_loss.py for coverage rate, test=develop

* rm comment in test_cross_entropy_loss.py, test=develop

* del  hard code of "float64" in python/paddle/nn/functional/loss.py, test=develop

* change mask to a more simplified implementation, test=develop

* del comment in python/paddle/nn/functional/loss.py, test=develop

* del hard code and change mask to a more simplified implementation, test=develop

* change mask to a more simplified implementation, test=develop

* change mask to a more simplified implementation, test=develop