Float64: CuDNN convolution backward does not work for float64
Cudnn backward convolution in https://github.com/mratsim/Arraymancer/pull/153 does not work for float64.

It triggers SIGSEV Illegal Storage access in both:

```Nim
  check cudnnConvolutionBackwardFilter(
    defaultHandle_cudnn,
    addr alpha,
    srcTensorDesc,
    input.data.data[],
    gradOutputTensorDesc,
    gOutput.data.data[],
    convDesc,
    kernel_algo_workspace.algo,
    kernel_algo_workspace.workspace[],
    kernel_algo_workspace.sizeInBytes,
    addr beta,
    gradKernelDesc,
    grad_kernel.data.data[]
  )
```

and

```Nim
  check cudnnConvolutionBackwardData(
    defaultHandle_cudnn,
    addr alpha,
    kernelDesc,
    kernel.data.data[],
    gradOutputTensorDesc,
    gOutput.data.data[],
    convDesc,
    gradInput_algo_workspace.algo,
    gradInput_algo_workspace.workspace[],
    gradInput_algo_workspace.sizeInBytes,
    addr beta,
    gradInputTensorDesc,
    grad_input.data.data[]
  )
```

