Feature request: Specifying GPU IDs

It would be ideal to specify the GPU IDs that a script can use. i typically like to have a GPU ID argument in my script so that I can set the device, but if Accelerate is handling the devices I can't specify. The current alternative is to use `CUDA_VISIBLE_DEVICES` but a dedicated argument in `accelerate config` or in the `Accelerator` object would be ideal.