Lazy object instantiation via config file

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### Description & Motivation

As far as I can see, there is no way currently to do lazy instantiation.

Let say, we want to create an object of class `path.to.B` that we configure from the yaml file. However, its creation might depend on a property of another object, that we know only during runtime. So it would be great if, based on the config file, we get a constructor for an object of class `path.to.B` , where all parameters are already set from the config file. In the example below, the constructor would already set `u=1, v=2`, e.g. using `functools.partial`. So instead of getting the instantiated object I get a preconfigured constructor.

See the example below

Module:

```python
class Model(pl.LightningModule):

def __init__(self,model):
super().__init__()
obj_A = model.A
obj_B_constructor = model.B
obj_B = obj_B_constructor(extern_param=obj_A.param) # should correspond to path.to.B_class(u=1, v=2, extern_param=obj_A)
```
config file:
```
model:
A:
class_path: path.to.A_class
init_args:
a: 1
b: 2
B:
class_path: path.to.B_class
lazy_init: True
init_args:
u: 1
v: 2
```

### Pitch

By setting an additional key in the config file, we are not getting the instantiate object, but the constructor for the object, with all parameters already set.

### Alternatives

_No response_

### Additional context

_No response_

cc @borda @carmocca @mauvilsa

Start by tracing how configuration entries with class_path and init_args are instantiated, then inspect the related tests if available. Define the expected behavior for lazy_init, including how the returned constructor accepts extern_param and how existing eager instantiation remains unchanged.

Written by the indexing model from the issue text.