How to find functions imported from torch._C in source code
I'm trying to track down the implementation of torch.nn.NLLLoss in the source code. I got as far as a call to torch._C.nll_loss in the function nll_loss in the file torch.nn.functional. But I can't find a place where _C is created.

Anyone have any info on this?  

Take a look at A Tour of PyTorch Internals on the PyTorch blog. Relevant excerpt:

PyTorch defines a new package torch. In this post we will consider the ._C module. This module is known as an “extension module” - a Python module written in C. Such modules allow us to define new built-in object types (e.g. the Tensor) and to call C/C++ functions.
The ._C module is defined in torch/csrc/Module.cpp. The init_C() / PyInit__C() function creates the module and adds the method definitions as appropriate. This module is passed around to a number of different __init() functions that add further objects to the module, register new types, etc.

Part II to that post goes into detail about the build system. In the section on NN modules, it says

Briefly, let’s touch on the last part of the build_deps command: generate_nn_wrappers(). We bind into the backend libraries using PyTorch’s custom cwrap tooling, which we touched upon in a previous post. For binding TH and THC we manually write the YAML declarations for each function. However, due to the relative simplicity of the THNN and THCUNN libraries, we auto-generate both the cwrap declarations and the resulting C++ code.
The reason we copy the THNN.h and THCUNN.h header files into torch/lib is that this is where the generate_nn_wrappers() code expects these files to be located. generate_nn_wrappers() does a few things:

Parses the header files, generating cwrap YAML declarations and writing them to output .cwrap files
Calls cwrap with the appropriate plugins on these .cwrap files to generate source code for each
Parses the headers a second time to generate THNN_generic.h - a library that takes THPP Tensors, PyTorch’s “generic” C++ Tensor Library, and calls into the appropriate THNN/THCUNN library function based on the dynamic type of the Tensor


Perhaps not that helpful without the context, but I don't think I should copy the entire post here.
When I tried to track down the definition of NLLLoss without having read those posts, I ended up at aten/src/THNN/generic/ClassNLLCriterion.c, via aten/src/ATen/nn.yaml. The latter is probably the YAML the second post talks about, but I haven't checked.
