`bench_cg.py` fails with NVRTC Error

Running bench_cg. py from the examples, gives me this error:

```
python bench_cg.py
ArrayFire v3.6.2 (CUDA, 64-bit Linux, build dc38ef1)
Platform: CUDA Toolkit 10, Driver: 410.79
[0] Tesla K80, 11442 MB, CUDA Compute 3.7
-1- Tesla K80, 11442 MB, CUDA Compute 3.7
-2- Tesla K80, 11442 MB, CUDA Compute 3.7
-3- Tesla K80, 11442 MB, CUDA Compute 3.7

Testing benchmark functions...
Traceback (most recent call last):
File "bench_cg.py", line 197, in
test()
File "bench_cg.py", line 144, in test
if af.sum(af.abs(x1 - x2)/x2 > 1e-6):
File "/home/vanderaa/miniconda3/envs/arrayfire/lib/python3.7/site-packages/arrayfire/array.py", line 894, in __truediv__
return _binary_func(self, other, backend.get().af_div)
File "/home/vanderaa/miniconda3/envs/arrayfire/lib/python3.7/site-packages/arrayfire/array.py", line 175, in _binary_func
safe_call(c_func(c_pointer(out.arr), lhs.arr, other.arr, _bcast_var.get()))
File "/home/vanderaa/miniconda3/envs/arrayfire/lib/python3.7/site-packages/arrayfire/util.py", line 79, in safe_call
raise RuntimeError(to_str(err_str))
RuntimeError: In function std::vector cuda::compileToPTX(const char*, std::string)
In file src/backend/cuda/jit.cpp:298
NVRTC Error(6): NVRTC_ERROR_COMPILATION
```

System: Linux K80 4.15.0-1035-azure #36~16.04.1-Ubuntu SMP Fri Nov 30 15:25:49 UTC 2018 x86_64 x86_64 x86_64 GNU/Linux
python --version:
Python 3.7.1
```
$pip show arrayfire
Name: arrayfire
Version: 3.6.20181017
Summary: Python bindings for ArrayFire
Home-page: http://arrayfire.com
Author: Pavan Yalamanchili
Author-email: contact@pavanky.com
License: BSD
Location: /mnt/vanderaa/miniconda3/envs/arrayfire/lib/python3.7/site-packages
Requires:
Required-by:
```

Start by running examples/bench_cg.py with the Python and ArrayFire versions reported in the issue, then inspect the failing operation at line 144 and the NVRTC error from src/backend/cuda/jit.cpp:298. Determine why the benchmark cannot compile this operation in the reported CUDA environment; done means the benchmark completes its tests without the NVRTC compilation error.

索引モデルが issue の本文から書いたものです。