C++ API parity: NLLLoss & CrossEntropyLoss (#29812)
Summary:
Hi yf225 , I have added **NLLLoss and CrossEntropyLoss.**
```
Also, while using log_softmax in cross_entropy_loss, I am getting an error
../caffe2/../torch/csrc/api/include/torch/nn/functional/loss.h:537:63: error: no matching function for call to log_softmax(const at::Tensor&)’
const Tensor& log_softmax_input = torch::log_softmax(input);
aten/src/ATen/Functions.h:5551:22: note: candidate: at::Tensor at::log_softmax(const at::Tensor&, int64_t, c10::optional<c10::ScalarType>)
static inline Tensor log_softmax(const Tensor & self, int64_t dim, c10::optional<ScalarType> dtype) {
^~~~~~~~~~~
aten/src/ATen/Functions.h:5551:22: note: candidate expects 3 arguments, 1 provided
```
I think the other two parameters should be optional as in python frontend(shown in documentation here at https://pytorch.org/docs/stable/nn.functional.html#torch.nn.functional.log_softmax ). Rest, there were no errors in build and tests have passed
Pull Request resolved: https://github.com/pytorch/pytorch/pull/29812
Differential Revision: D18548249
Pulled By: yf225
fbshipit-source-id: 2ab350abd2a6f498d4dba2345f51ad87471f3038
