support multiple devices per backend

Author: vchuravy

src/KernelAbstractions.jl

@@ -546,6 +546,33 @@ function priority!(::Backend, prio::Symbol)
     return nothing
 end
 
+"""
+    device(::Backend)
+
+Returns the ordinal number of the currently active device starting at one.
+"""
+function device(::Backend)
+    return 1
+end
+
+"""
+    ndevices(::Backend)
+
+Returns the number of devices the backend supports.
+"""
+function ndevices(::Backend)
+    return 1
+end
+
+"""
+    device!(::Backend, id::Int)
+"""
+function device!(backend::Backend, id::Int)
+    if 0 < id <= ndevices(backend)
+        throw(ArgumentError("Device id $id out of bounds."))
+    return nothing
+end
+
 """
     functional(::Backend)