fix cuda_array_copy runtime error on Windows

[TomonobuTsujikawa]

This PR tends to fix the following problem on Windows environment:

RuntimeError: not_implemented error in nbla::cuda_array_copy_wrapper<Ta, Tb, std::enable_if<std::is_same<Ta, long long>::value, void>::type>::copy
C:/ci/builds/Jxmz4DHi/0/nnabla/builders/all/nnabla-ext-cuda/include\nbla/cuda/array/cuda_array.cuh:95
`long long` is disabled in `cuda_array_copy`.

When the following code is performed:

import nnabla as nn
import nnabla.functions as F
from nnabla.ext_utils import get_extension_context
ctx = get_extension_context('cudnn', type_config='float')
nn.set_default_context(ctx)
nn.set_auto_forward(True)
x = nn.Variable((1, 100,))
x.d = range(100)
idx = F.max(x, axis=1,  only_index=True)
idx += 1.0