backend: Don't return an error on Close

It is not common to return errors with close/free operations - most
people won't check it and even if they did there's probably not much
that can do. It's better to not give implementations false expectations.

ml/backend.go

@@ -50,7 +50,7 @@ type Context interface {
 
 	Forward(Tensor)
 	Compute(Tensor) Tensor
-	Close() error
+	Close()
 }
 
 type Tensor interface {