Add default dtype to backend

include/backend.hpp

@@ -1,8 +1,10 @@
 #pragma once
 
 #include <memory>
+#include <optional>
 
 #include "shape.hpp"
+#include "tensor.hpp"
 
 namespace CUDANet {
 
@@ -22,7 +24,14 @@ class BackendFactory {
 };
 
 class Backend {
+  protected:
+    std::optional<DType> default_dtype;
   public:
+
+    virtual bool supports_dtype(DType dtype) const = 0;
+    virtual void set_default_dtype(DType dtype) = 0;
+    virtual DType get_default_dtype() const = 0;
+
     // Memory management
     virtual void* allocate(size_t bytes) = 0;
     virtual void  deallocate(void* ptr)  = 0;