Namespace gets flattened if under a nested key

[carmocca]

🐛 Bug report

Traceback (most recent call last):
  File "/home/ubuntu/carlos/kk.py", line 18, in <module>
    args = parser.instantiate_classes(args)
  File "/home/ubuntu/.pyenv/versions/env/lib/python3.10/site-packages/jsonargparse/_deprecated.py", line 140, in patched_instantiate_classes
    cfg = self._unpatched_instantiate_classes(cfg, **kwargs)
  File "/home/ubuntu/.pyenv/versions/env/lib/python3.10/site-packages/jsonargparse/_core.py", line 1207, in instantiate_classes
    component.instantiate_class(component, cfg)
  File "/home/ubuntu/.pyenv/versions/env/lib/python3.10/site-packages/jsonargparse/_signatures.py", line 560, in group_instantiate_class
    parent[key] = instantiator_fn(group.group_class, **value)
  File "/home/ubuntu/.pyenv/versions/env/lib/python3.10/site-packages/jsonargparse/_common.py", line 155, in default_class_instantiator
    return class_type(*args, **kwargs)
TypeError: MyConfig.__init__() got an unexpected keyword argument 'optimizer.class_path'

To reproduce

from dataclasses import dataclass
from jsonargparse import ArgumentParser, Namespace
import torch

@dataclass
class Foo:
    f: int

@dataclass
class MyConfig:
    f: Foo
    optimizer: Namespace

parser = ArgumentParser()
parser.add_class_arguments(MyConfig, nested_key="my_config", skip={"optimizer"})
parser.add_subclass_arguments((torch.optim.Optimizer,), nested_key="my_config.optimizer", skip={"params"}, instantiate=False)
args = parser.parse_args(args=["--my_config.f.f=123", "--my_config.optimizer=torch.optim.Adam"])
args = parser.instantiate_classes(args)
print(args)

This works though (there is no nested key):

from dataclasses import dataclass
from jsonargparse import ArgumentParser, Namespace
import torch

@dataclass
class Foo:
    f: int

@dataclass
class MyConfig:
    f: Foo
    optimizer: Namespace

parser = ArgumentParser()
parser.add_class_arguments(MyConfig, skip={"optimizer"})
parser.add_subclass_arguments((torch.optim.Optimizer,), nested_key="optimizer", skip={"params"}, instantiate=False)
args = parser.parse_args(args=["--f.f=123", "--optimizer=torch.optim.Adam"])
args = parser.instantiate_classes(args)
print(args)

Expected behavior

Prints

Namespace(my_config=Namespace(f=Foo(f=123), optimizer=Namespace(class_path='torch.optim.Adam', init_args=Namespace(lr=0.001, betas=(0.9, 0.999), eps=1e-08, weight_decay=0.0, amsgrad=False, foreach=None, maximize=False, capturable=False, differentiable=False, fused=None))))

Environment

• jsonargparse version: 4.35.0
• Python version: 3.10.12
• How jsonargparse was installed: pip
• OS: Linux

Merry christmas