Add some skeleton code for GPU support.

src/device.rs

@@ -6,6 +6,7 @@ use crate::{
 #[derive(Debug, Copy, Clone, PartialEq, Eq, Hash)]
 pub enum Device {
     Cpu,
+    Cuda { gpu_id: usize },
 }
 
 // TODO: Should we back the cpu implementation using the NdArray crate or similar?
@@ -72,6 +73,9 @@ impl Device {
                 };
                 Storage::Cpu(storage)
             }
+            Device::Cuda { gpu_id: _ } => {
+                todo!()
+            }
         }
     }
 
@@ -91,12 +95,18 @@ impl Device {
                 };
                 Storage::Cpu(storage)
             }
+            Device::Cuda { gpu_id: _ } => {
+                todo!()
+            }
         }
     }
 
     pub(crate) fn tensor<A: NdArray>(&self, array: A) -> Storage {
         match self {
             Device::Cpu => Storage::Cpu(array.to_cpu_storage()),
+            Device::Cuda { gpu_id: _ } => {
+                todo!()
+            }
         }
     }
 }