Error in Apex's FusedLayerNorm
After installing `apex` with the cuda extensions and running BERT, I get the following error in `FusedLayerNormAffineFunction`, [apex/normalization/fused_layer_norm.py](https://github.com/NVIDIA/apex/blob/master/apex/normalization/fused_layer_norm.py#L16) (line 21).

```
RuntimeError: a Tensor with 2482176 elements cannot be converted to Scalar (item at /pytorch/aten/src/ATen/native/Scalar.cpp:9)
```

Here are the shapes of my tensors:

```
input_ - [32, 101, 768]  (this is the embeddings tensor in BertEmbeddings)
bias_ - [768]
weight_ - [768]
self.normalized_shape - [768]
```

I'm not sure if it's a problem with `pytorch-pretrained-BERT` or `apex`. Any idea?

Full stacktrace below.

```
File "/home/hyper/Documents/anaconda3/envs/allennlp/lib/python3.6/site-packages/pytorch_pretrained_bert/modeling.py", line 710, in forward
    embedding_output = self.embeddings(input_ids, token_type_ids)
  File "/home/hyper/Documents/anaconda3/envs/allennlp/lib/python3.6/site-packages/torch/nn/modules/module.py", line 489, in __call__
    result = self.forward(*input, **kwargs)
  File "/home/hyper/Documents/anaconda3/envs/allennlp/lib/python3.6/site-packages/pytorch_pretrained_bert/modeling.py", line 261, in forward
    embeddings = self.LayerNorm(embeddings)
  File "/home/hyper/Documents/anaconda3/envs/allennlp/lib/python3.6/site-packages/torch/nn/modules/module.py", line 489, in __call__
    result = self.forward(*input, **kwargs)
  File "/home/hyper/Documents/anaconda3/envs/allennlp/lib/python3.6/site-packages/apex-0.1-py3.6-linux-x86_64.egg/apex/normalization/fused_layer_norm.py", line 149, in forward
    input, self.weight, self.bias)
  File "/home/hyper/Documents/anaconda3/envs/allennlp/lib/python3.6/site-packages/apex-0.1-py3.6-linux-x86_64.egg/apex/normalization/fused_layer_norm.py", line 21, in forward
    input_, self.normalized_shape, weight_, bias_, self.eps)

RuntimeError: a Tensor with 2482176 elements cannot be converted to Scalar (item at /pytorch/aten/src/ATen/native/Scalar.cpp:9)

frame #0: std::function<std::string ()>::operator()() const + 0x11 (0x7f1aa5da3021 in /home/hyper/Documents/anaconda3/envs/allennlp/lib/python3.6/site-packages/torch/lib/libc10.so)
frame #1: c10::Error::Error(c10::SourceLocation, std::string const&) + 0x2a (0x7f1aa5da28ea in /home/hyper/Documents/anaconda3/envs/allennlp/lib/python3.6/site-packages/torch/lib/libc10.so)
frame #2: at::native::item(at::Tensor const&) + 0x12c3 (0x7f1aa690d5b3 in /home/hyper/Documents/anaconda3/envs/allennlp/lib/python3.6/site-packages/torch/lib/libcaffe2.so)
frame #3: at::TypeDefault::item(at::Tensor const&) const + 0x55 (0x7f1aa6b1c905 in /home/hyper/Documents/anaconda3/envs/allennlp/lib/python3.6/site-packages/torch/lib/libcaffe2.so)
frame #4: torch::autograd::VariableType::eye_out(at::Tensor&, long, long) const + 0x184 (0x7f1aa4faeec4 in /home/hyper/Documents/anaconda3/envs/allennlp/lib/python3.6/site-packages/torch/lib/libtorch.so.1)
frame #5: <unknown function> + 0x89ca (0x7f1a82e739ca in /home/hyper/Documents/anaconda3/envs/allennlp/lib/python3.6/site-packages/apex-0.1-py3.6-linux-x86_64.egg/fused_layer_norm_cuda.cpython-36m-x86_64-linux-gnu.so)
frame #6: layer_norm_affine(at::Tensor, c10::ArrayRef<long>, at::Tensor, at::Tensor, double) + 0x185 (0x7f1a82e762a5 in /home/hyper/Documents/anaconda3/envs/allennlp/lib/python3.6/site-packages/apex-0.1-py3.6-linux-x86_64.egg/fused_layer_norm_cuda.cpython-36m-x86_64-linux-gnu.so)
frame #7: <unknown function> + 0x18d44 (0x7f1a82e83d44 in /home/hyper/Documents/anaconda3/envs/allennlp/lib/python3.6/site-packages/apex-0.1-py3.6-linux-x86_64.egg/fused_layer_norm_cuda.cpython-36m-x86_64-linux-gnu.so)
frame #8: <unknown function> + 0x16495 (0x7f1a82e81495 in /home/hyper/Documents/anaconda3/envs/allennlp/lib/python3.6/site-packages/apex-0.1-py3.6-linux-x86_64.egg/fused_layer_norm_cuda.cpython-36m-x86_64-linux-gnu.so)
frame #9: _PyCFunction_FastCallDict + 0x154 (0x55a8f9925744 in /home/hyper/Documents/anaconda3/envs/allennlp/bin/python)
frame #10: <unknown function> + 0x198610 (0x55a8f99ac610 in /home/hyper/Documents/anaconda3/envs/allennlp/bin/python)
frame #11: _PyEval_EvalFrameDefault + 0x30a (0x55a8f99d138a in /home/hyper/Documents/anaconda3/envs/allennlp/bin/python)
frame #12: <unknown function> + 0x71e1 (0x7f1af51ee1e1 in /home/hyper/.PyCharm2018.1/system/cythonExtensions/_pydevd_frame_eval_ext/pydevd_frame_evaluator.cpython-36m-x86_64-linux-gnu.so)
frame #13: _PyFunction_FastCallDict + 0x11b (0x55a8f99a6bab in /home/hyper/Documents/anaconda3/envs/allennlp/bin/python)
frame #14: _PyObject_FastCallDict + 0x26f (0x55a8f9925b0f in /home/hyper/Documents/anaconda3/envs/allennlp/bin/python)
frame #15: _PyObject_Call_Prepend + 0x63 (0x55a8f992a6a3 in /home/hyper/Documents/anaconda3/envs/allennlp/bin/python)
frame #16: PyObject_Call + 0x3e (0x55a8f992554e in /home/hyper/Documents/anaconda3/envs/allennlp/bin/python)
frame #17: THPFunction_do_forward(THPFunction*, _object*) + 0x15c (0x7f1ae02e21ec in /home/hyper/Documents/anaconda3/envs/allennlp/lib/python3.6/site-packages/torch/lib/libtorch_python.so)
frame #18: PyCFunction_Call + 0x5f (0x55a8f992863f in /home/hyper/Documents/anaconda3/envs/allennlp/bin/python)
frame #19: PyObject_Call + 0x3e (0x55a8f992554e in /home/hyper/Documents/anaconda3/envs/allennlp/bin/python)
frame #20: <unknown function> + 0x16ba91 (0x55a8f997fa91 in /home/hyper/Documents/anaconda3/envs/allennlp/bin/python)
frame #21: _PyObject_FastCallDict + 0x8b (0x55a8f992592b in /home/hyper/Documents/anaconda3/envs/allennlp/bin/python)
frame #22: <unknown function> + 0x19857e (0x55a8f99ac57e in /home/hyper/Documents/anaconda3/envs/allennlp/bin/python)
frame #23: _PyEval_EvalFrameDefault + 0x30a (0x55a8f99d138a in /home/hyper/Documents/anaconda3/envs/allennlp/bin/python)
frame #24: <unknown function> + 0x71e1 (0x7f1af51ee1e1 in /home/hyper/.PyCharm2018.1/system/cythonExtensions/_pydevd_frame_eval_ext/pydevd_frame_evaluator.cpython-36m-x86_64-linux-gnu.so)
frame #25: _PyFunction_FastCallDict + 0x11b (0x55a8f99a6bab in /home/hyper/Documents/anaconda3/envs/allennlp/bin/python)
frame #26: _PyObject_FastCallDict + 0x26f (0x55a8f9925b0f in /home/hyper/Documents/anaconda3/envs/allennlp/bin/python)
frame #27: _PyObject_Call_Prepend + 0x63 (0x55a8f992a6a3 in /home/hyper/Documents/anaconda3/envs/allennlp/bin/python)
frame #28: PyObject_Call + 0x3e (0x55a8f992554e in /home/hyper/Documents/anaconda3/envs/allennlp/bin/python)
frame #29: _PyEval_EvalFrameDefault + 0x19ec (0x55a8f99d2a6c in /home/hyper/Documents/anaconda3/envs/allennlp/bin/python)
frame #30: <unknown function> + 0x71e1 (0x7f1af51ee1e1 in /home/hyper/.PyCharm2018.1/system/cythonExtensions/_pydevd_frame_eval_ext/pydevd_frame_evaluator.cpython-36m-x86_64-linux-gnu.so)
frame #31: <unknown function> + 0x1918e4 (0x55a8f99a58e4 in /home/hyper/Documents/anaconda3/envs/allennlp/bin/python)
frame #32: _PyFunction_FastCallDict + 0x1bc (0x55a8f99a6c4c in /home/hyper/Documents/anaconda3/envs/allennlp/bin/python)
frame #33: _PyObject_FastCallDict + 0x26f (0x55a8f9925b0f in /home/hyper/Documents/anaconda3/envs/allennlp/bin/python)
frame #34: _PyObject_Call_Prepend + 0x63 (0x55a8f992a6a3 in /home/hyper/Documents/anaconda3/envs/allennlp/bin/python)
frame #35: PyObject_Call + 0x3e (0x55a8f992554e in /home/hyper/Documents/anaconda3/envs/allennlp/bin/python)
frame #36: <unknown function> + 0x16ba91 (0x55a8f997fa91 in /home/hyper/Documents/anaconda3/envs/allennlp/bin/python)
frame #37: _PyObject_FastCallDict + 0x8b (0x55a8f992592b in /home/hyper/Documents/anaconda3/envs/allennlp/bin/python)
frame #38: <unknown function> + 0x19857e (0x55a8f99ac57e in /home/hyper/Documents/anaconda3/envs/allennlp/bin/python)
frame #39: _PyEval_EvalFrameDefault + 0x30a (0x55a8f99d138a in /home/hyper/Documents/anaconda3/envs/allennlp/bin/python)
frame #40: <unknown function> + 0x71e1 (0x7f1af51ee1e1 in /home/hyper/.PyCharm2018.1/system/cythonExtensions/_pydevd_frame_eval_ext/pydevd_frame_evaluator.cpython-36m-x86_64-linux-gnu.so)
frame #41: <unknown function> + 0x1918e4 (0x55a8f99a58e4 in /home/hyper/Documents/anaconda3/envs/allennlp/bin/python)
frame #42: _PyFunction_FastCallDict + 0x3da (0x55a8f99a6e6a in /home/hyper/Documents/anaconda3/envs/allennlp/bin/python)
frame #43: _PyObject_FastCallDict + 0x26f (0x55a8f9925b0f in /home/hyper/Documents/anaconda3/envs/allennlp/bin/python)
frame #44: _PyObject_Call_Prepend + 0x63 (0x55a8f992a6a3 in /home/hyper/Documents/anaconda3/envs/allennlp/bin/python)
frame #45: PyObject_Call + 0x3e (0x55a8f992554e in /home/hyper/Documents/anaconda3/envs/allennlp/bin/python)
frame #46: _PyEval_EvalFrameDefault + 0x19ec (0x55a8f99d2a6c in /home/hyper/Documents/anaconda3/envs/allennlp/bin/python)
frame #47: <unknown function> + 0x71e1 (0x7f1af51ee1e1 in /home/hyper/.PyCharm2018.1/system/cythonExtensions/_pydevd_frame_eval_ext/pydevd_frame_evaluator.cpython-36m-x86_64-linux-gnu.so)
frame #48: <unknown function> + 0x1918e4 (0x55a8f99a58e4 in /home/hyper/Documents/anaconda3/envs/allennlp/bin/python)
frame #49: _PyFunction_FastCallDict + 0x1bc (0x55a8f99a6c4c in /home/hyper/Documents/anaconda3/envs/allennlp/bin/python)
frame #50: _PyObject_FastCallDict + 0x26f (0x55a8f9925b0f in /home/hyper/Documents/anaconda3/envs/allennlp/bin/python)
frame #51: _PyObject_Call_Prepend + 0x63 (0x55a8f992a6a3 in /home/hyper/Documents/anaconda3/envs/allennlp/bin/python)
frame #52: PyObject_Call + 0x3e (0x55a8f992554e in /home/hyper/Documents/anaconda3/envs/allennlp/bin/python)
frame #53: <unknown function> + 0x16ba91 (0x55a8f997fa91 in /home/hyper/Documents/anaconda3/envs/allennlp/bin/python)
frame #54: _PyObject_FastCallDict + 0x8b (0x55a8f992592b in /home/hyper/Documents/anaconda3/envs/allennlp/bin/python)
frame #55: <unknown function> + 0x19857e (0x55a8f99ac57e in /home/hyper/Documents/anaconda3/envs/allennlp/bin/python)
frame #56: _PyEval_EvalFrameDefault + 0x30a (0x55a8f99d138a in /home/hyper/Documents/anaconda3/envs/allennlp/bin/python)
frame #57: <unknown function> + 0x71e1 (0x7f1af51ee1e1 in /home/hyper/.PyCharm2018.1/system/cythonExtensions/_pydevd_frame_eval_ext/pydevd_frame_evaluator.cpython-36m-x86_64-linux-gnu.so)
frame #58: <unknown function> + 0x1918e4 (0x55a8f99a58e4 in /home/hyper/Documents/anaconda3/envs/allennlp/bin/python)
frame #59: _PyFunction_FastCallDict + 0x3da (0x55a8f99a6e6a in /home/hyper/Documents/anaconda3/envs/allennlp/bin/python)
frame #60: _PyObject_FastCallDict + 0x26f (0x55a8f9925b0f in /home/hyper/Documents/anaconda3/envs/allennlp/bin/python)
frame #61: _PyObject_Call_Prepend + 0x63 (0x55a8f992a6a3 in /home/hyper/Documents/anaconda3/envs/allennlp/bin/python)
frame #62: PyObject_Call + 0x3e (0x55a8f992554e in /home/hyper/Documents/anaconda3/envs/allennlp/bin/python)
frame #63: _PyEval_EvalFrameDefault + 0x19ec (0x55a8f99d2a6c in /home/hyper/Documents/anaconda3/envs/allennlp/bin/python)
```

This was an error in `apex`, due to mismatched compiled libraries. Fix can be found [here](https://github.com/NVIDIA/apex/issues/156#issuecomment-465301976).

> Try a full `pip uninstall apex`, then `cd apex_repo_dir; rm-rf build; python setup.py install --cuda_ext --cpp_ext` and see if the segfault persists.