Fix magnitude cuda (#398)

* add missing fmt_left

* add comments

crates/cubecl-core/src/frontend/element/base.rs

@@ -31,6 +31,8 @@ pub trait CubeType {
 }
 
 /// Trait useful for cube types that are also used with comptime.
+///
+/// This is used to set a variable as mutable. (Need to be fixed or at least renamed.)
 pub trait IntoRuntime: CubeType + Sized {
     /// Make sure a type is actually expanded into its runtime [expand type](CubeType::ExpandType).
     fn runtime(self) -> Self {

crates/cubecl-core/src/frontend/operation/base.rs

@@ -187,15 +187,13 @@ where
     if input.can_mut() {
         return input;
     }
-
     let input_var: Variable = *input;
     let item = input.item;
 
-    let out = context.create_local_mut(item);
+    let out = context.create_local_mut(item); // TODO: The mut is safe, but unecessary if the variable is immutable.
     let out_var = *out;
 
     let op = func(input_var);
-
     context.register(Instruction::new(op, out_var));
 
     out

crates/cubecl-cpp/src/shared/instruction.rs

@@ -764,6 +764,7 @@ impl<D: Dialect> Magnitude<D> {
             writeln!(f, "{mag} += {input_i} * {input_i};")?;
         }
 
+        let out = out.fmt_left();
         write!(f, "{out} = ")?;
         Sqrt::format_unary(f, &mag, elem)?;
         f.write_str(";\n")