Fix TensorAlloc copy that was copying the ptr

src/TensorAlloc.zig

@@ -60,9 +60,9 @@ pub fn Tensor(
         }
 
         pub fn copy(self: *const Self, alloc: Allocator) !Self {
-            var new = try splat(alloc, 0);
-            new.data = self.data;
-            return new;
+            const vec_ptr = try alloc.create(Vec);
+            vec_ptr.* = self.data.*;
+            return .{ .data = vec_ptr };
         }
 
         /// Convert N-D coords (row-major) to flat index — fully comptime.