How to specify device for Metal Compute in CIImageProcessorKernel subclass?

I've got a custom CIFilter, implemented using a subclass of CIImageProcessorKernel. The kernel itself is very simple:

@implementation ErosionFilterKernel

+ (BOOL)processWithInputs:(NSArray<id<CIImageProcessorInput>> *)inputs
                arguments:(NSDictionary<NSString *,id> *)arguments
                   output:(id<CIImageProcessorOutput>)output
                    error:(NSError *__autoreleasing *)error
{
    error = error ?: &(NSError * __autoreleasing){ nil };

    id<MTLCommandBuffer> commandBuffer = output.metalCommandBuffer;
    id<MTLTexture> sourceTexture = [inputs.firstObject metalTexture];
    id<MTLTexture> destinationTexture = output.metalTexture;

    NSInteger distance = [arguments[@"erosionDistance"] integerValue] ?: 1;

    MPSImageAreaMin *erodeFilter = [[MPSImageAreaMin alloc] initWithDevice:commandBuffer.device
                                                               kernelWidth:distance
                                                              kernelHeight:distance];
    [erodeFilter encodeToCommandBuffer:commandBuffer sourceTexture:sourceTexture destinationTexture:destinationTexture];
    return YES;
}

@end

This works fine, in that it produces the expected result. The problem I'm having is that it uses the integrated GPU on a MacBook Pro with two GPUs, and I'd like it to use the discrete GPU. If I pass the result of MTLCreateSystemDefaultDevice() (the discrete GPU) into -[MPSImageAreaMin initWithDevice:...], I get an assertion failure:

-[MTLDebugComputeCommandEncoder setComputePipelineState:] failed assertion computePipelineState is associated with a different device

This is presumably because the instance of MTLComputeCommandEncoder used internally by the machinery responsible for running -encodeToCommandBuffer:sourceTexture:destinationTexture: has already been set up to use the integrated GPU. I think this comes from the commandBuffer pulled from the CIImageProcessorOutput object.

My question: Is it possible to specify the GPU to be used by -encodeToCommandBuffer:sourceTexture:destinationTexture:? Presumably that involves customizing the output/metal command buffer, but I'm not sure about that.

Which GPU is used should be determined by the CIContext that performs the image processing. You should be able to specify the device by using the [CIContext contextWithMTLDevice:] initializer.

By the way, since macOS 10.15 there's also a built-in CIMorphologyRectangleMinimum filter that does the same. And there's also a circular version that's available since 10.13.