use factories in context example

Using the same instance multiple times is a bad idea
because PyTorch memorizes things internally. Among
other things this breaks Chain's `__repr__`.

docs/concepts/context.md

@@ -53,22 +53,22 @@ Another use of the context is simplifying complex models, in particular those wi
 To emulate this, let us consider this toy example with a structure somewhat similar to a U-Net:
 
 ```py
-square = fl.Lambda(lambda x: x ** 2)
-sqrt = fl.Lambda(lambda x: x ** 0.5)
+square = lambda: fl.Lambda(lambda x: x ** 2)
+sqrt = lambda: fl.Lambda(lambda x: x ** 0.5)
 
 m1 = fl.Chain(
     fl.Residual(
-        square,
+        square(),
         fl.Residual(
-            square,
+            square(),
             fl.Residual(
-                square,
+                square(),
             ),
-            sqrt,
+            sqrt(),
         ),
-        sqrt,
+        sqrt(),
     ),
-    sqrt,
+    sqrt(),
 )
 ```
 
@@ -86,7 +86,12 @@ class MyModel(fl.Chain):
     def init_context(self) -> Contexts:
         return {"mymodel": {"residuals": []}}
 
-push_residual = fl.SetContext("mymodel", "residuals", callback=lambda l, x: l.append(x))
+def push_residual():
+    return fl.SetContext(
+        "mymodel",
+        "residuals",
+        callback=lambda l, x: l.append(x),
+    )
 
 class ApplyResidual(fl.Sum):
     def __init__(self):
@@ -95,8 +100,8 @@ class ApplyResidual(fl.Sum):
             fl.UseContext("mymodel", "residuals").compose(lambda x: x.pop()),
         )
 
-squares = fl.Chain(x for _ in range(3) for x in (push_residual, square))
-sqrts = fl.Chain(x for _ in range(3) for x in (ApplyResidual(), sqrt))
+squares = fl.Chain(x for _ in range(3) for x in (push_residual(), square()))
+sqrts = fl.Chain(x for _ in range(3) for x in (ApplyResidual(), sqrt()))
 m2 = MyModel(squares, sqrts)
 ```