fix clang-tidy
fix clang-tidy

xchainer/cuda/cuda_device.cc:13:9: error: implicit cast 'cublasHandle_t' (aka 'cublasContext *') -> bool [readability-implicit-bool-cast,-warnings-as-errors]
    if (cublas_handle_) {
        ^             ~
                       != nullptr
xchainer/cuda/cuda_device.cc:20:10: error: implicit cast 'cublasHandle_t' (aka 'cublasContext *') -> bool [readability-implicit-bool-cast,-warnings-as-errors]
    if (!cublas_handle_) {
        ~^~~~~~~~~~~~~~~
         == nullptr

hvy/chainer,okuta/chainer,chainer/chainer,ktnyt/chainer,wkentaro/chainer,okuta/chainer,keisuke-umezawa/chainer,chainer/chainer,hvy/chainer,jnishi/chainer,hvy/chainer,jnishi/chainer,wkentaro/chainer,jnishi/chainer,wkentaro/chainer,jnishi/chainer,chainer/chainer,chainer/chainer,okuta/chainer,keisuke-umezawa/chainer,ktnyt/chainer,niboshi/chainer,okuta/chainer,keisuke-umezawa/chainer,wkentaro/chainer,ktnyt/chainer,tkerola/chainer,pfnet/chainer,hvy/chainer,ktnyt/chainer,niboshi/chainer,niboshi/chainer,niboshi/chainer,keisuke-umezawa/chainer