Access instance attributes in config [Enhancement]

Say we have a setup like this

```python
class A:
def __init___(self, x, y):
# some value computed from the parameters
self.n = x ** 2 + y

class B:
def __init___(self, n):
self.n = n
```

I would like to be able to do this in my config:

```python
A.x = 3
A.y = 2
B.n = @A().n
```

Hard-coding `B.n = 11` would be bad because we might forget to change it when we update `A.x`. Obviously one can just not use the config for this and create `a = A()` first in Python then `b = B(a.n)`. If the instances of `A` and `B` wouldn't naturally be created in the same place, though, then this makes the code worse since we have to pass `a` (or at least `a.n`) around.