omegaconf does not resolve types before using

[ssnl]

Describe the bug
When type annotation is a str (e.g., to work around some forward reference), omegaconf does not resolve them before using, leading to various bugs.

To Reproduce

from omegaconf import OmegaConf
import attrs 

@attrs.define
class C:
    @attrs.define
    class CA:
        x: int = 3
        
    @attrs.define
    class CB:
        sub: 'C.CA' = attrs.field()
        
    a: CA 
    b: CB


obj = C(a=C.CA(), b=C.CB(C.CA(x=33)))
OmegaConf.create(obj)

Error is

...
File ~/miniconda3/lib/python3.8/site-packages/omegaconf/dictconfig.py:649, in DictConfig._set_value(self, value, flags)
    [647](file:///Users/S_sn/miniconda3/lib/python3.8/site-packages/omegaconf/dictconfig.py?line=646) except Exception as e:
    [648](file:///Users/S_sn/miniconda3/lib/python3.8/site-packages/omegaconf/dictconfig.py?line=647)     self.__dict__["_content"] = previous_content
--> [649](file:///Users/S_sn/miniconda3/lib/python3.8/site-packages/omegaconf/dictconfig.py?line=648)     raise e

File ~/miniconda3/lib/python3.8/site-packages/omegaconf/dictconfig.py:646, in DictConfig._set_value(self, value, flags)
    [644](file:///Users/S_sn/miniconda3/lib/python3.8/site-packages/omegaconf/dictconfig.py?line=643) try:
    [645](file:///Users/S_sn/miniconda3/lib/python3.8/site-packages/omegaconf/dictconfig.py?line=644)     previous_content = self.__dict__["_content"]
--> [646](file:///Users/S_sn/miniconda3/lib/python3.8/site-packages/omegaconf/dictconfig.py?line=645)     self._set_value_impl(value, flags)
    [647](file:///Users/S_sn/miniconda3/lib/python3.8/site-packages/omegaconf/dictconfig.py?line=646) except Exception as e:
    [648](file:///Users/S_sn/miniconda3/lib/python3.8/site-packages/omegaconf/dictconfig.py?line=647)     self.__dict__["_content"] = previous_content

File ~/miniconda3/lib/python3.8/site-packages/omegaconf/dictconfig.py:660, in DictConfig._set_value_impl(self, value, flags)
    [657](file:///Users/S_sn/miniconda3/lib/python3.8/site-packages/omegaconf/dictconfig.py?line=656)     flags = {}
    [659](file:///Users/S_sn/miniconda3/lib/python3.8/site-packages/omegaconf/dictconfig.py?line=658) assert not isinstance(value, ValueNode)
--> [660](file:///Users/S_sn/miniconda3/lib/python3.8/site-packages/omegaconf/dictconfig.py?line=659) self._validate_set(key=None, value=value)
    [662](file:///Users/S_sn/miniconda3/lib/python3.8/site-packages/omegaconf/dictconfig.py?line=661) if _is_none(value, resolve=True):
    [663](file:///Users/S_sn/miniconda3/lib/python3.8/site-packages/omegaconf/dictconfig.py?line=662)     self.__dict__["_content"] = None

File ~/miniconda3/lib/python3.8/site-packages/omegaconf/dictconfig.py:206, in DictConfig._validate_set(self, key, value)
    [197](file:///Users/S_sn/miniconda3/lib/python3.8/site-packages/omegaconf/dictconfig.py?line=196) if is_container_annotation(target_type) and not is_container_annotation(
    [198](file:///Users/S_sn/miniconda3/lib/python3.8/site-packages/omegaconf/dictconfig.py?line=197)     value_type
    [199](file:///Users/S_sn/miniconda3/lib/python3.8/site-packages/omegaconf/dictconfig.py?line=198) ):
    [200](file:///Users/S_sn/miniconda3/lib/python3.8/site-packages/omegaconf/dictconfig.py?line=199)     raise ValidationError(
    [201](file:///Users/S_sn/miniconda3/lib/python3.8/site-packages/omegaconf/dictconfig.py?line=200)         f"Cannot assign {type_str(value_type)} to {type_str(target_type)}"
    [202](file:///Users/S_sn/miniconda3/lib/python3.8/site-packages/omegaconf/dictconfig.py?line=201)     )
    [203](file:///Users/S_sn/miniconda3/lib/python3.8/site-packages/omegaconf/dictconfig.py?line=202) validation_error = (
    [204](file:///Users/S_sn/miniconda3/lib/python3.8/site-packages/omegaconf/dictconfig.py?line=203)     target_type is not None
    [205](file:///Users/S_sn/miniconda3/lib/python3.8/site-packages/omegaconf/dictconfig.py?line=204)     and value_type is not None
--> [206](file:///Users/S_sn/miniconda3/lib/python3.8/site-packages/omegaconf/dictconfig.py?line=205)     and not issubclass(value_type, target_type)
    [207](file:///Users/S_sn/miniconda3/lib/python3.8/site-packages/omegaconf/dictconfig.py?line=206) )
    [208](file:///Users/S_sn/miniconda3/lib/python3.8/site-packages/omegaconf/dictconfig.py?line=207) if validation_error:
    [209](file:///Users/S_sn/miniconda3/lib/python3.8/site-packages/omegaconf/dictconfig.py?line=208)     self._raise_invalid_value(value, value_type, target_type)

ConfigTypeError: issubclass() arg 2 must be a class or tuple of classes
    full_key: sub
    object_type=None

caused by having a str ref_type.

Expected behavior
Arguably, omegaconf should use https://docs.python.org/3/library/typing.html#typing.get_type_hints to get the concrete type.

Additional context

• OmegaConf version: 2.2.1
• Python version: 3.7
• Operating system: mac
• Please provide a minimal repro

[Jasha10]

Arguably, omegaconf should use https://docs.python.org/3/library/typing.html#typing.get_type_hints to get the concrete type.

👍🏼 Good idea.