Adding c10 device type to newly added DeviceAccelerator (pytorch#119961)

Follow up to pytorch#104364,

A new file got submitted yesterday that is using DeviceType without the c10 namespace. This fixes that. I haven't yet figured out a way to setup a test for this, but I will submit a follow up PR once I figure that out.
Pull Request resolved: pytorch#119961
Approved by: https://github.com/ezyang

Author: anthonyalayo

aten/src/ATen/DeviceAccelerator.h

@@ -22,6 +22,6 @@ namespace at {
 // Ensures that only one accelerator is available (at
 // compile time if possible) and return it.
 // When checked is true, the returned optional always has a value.
-TORCH_API std::optional<DeviceType> getAccelerator(bool checked = false);
+TORCH_API std::optional<c10::DeviceType> getAccelerator(bool checked = false);
 
 } // namespace at