[Bug] instantiate: top-level _target_ has inconsistent behavior with _convert_

[Jasha10]

🐛 Bug

Description

The _convert_ flag has no effect when instantiating a config that has a _target_ at the top level.

To reproduce

from dataclasses import dataclass

import hydra
from omegaconf import DictConfig
from pytest import raises

@dataclass
class MyDataclass:
    ...

cfg = {"_target_": "__main__.MyDataclass"}

obj = hydra.utils.instantiate(cfg, _convert_="none")
with raises(AssertionError):
    assert isinstance(obj, DictConfig)  # got MyDataclass

obj = hydra.utils.instantiate(cfg, _convert_="partial")
with raises(AssertionError):
    assert isinstance(obj, DictConfig)  # got MyDataclass

obj = hydra.utils.instantiate(cfg, _convert_="object")
assert isinstance(obj, MyDataclass)  # Ok

obj = hydra.utils.instantiate(cfg, _convert_="all")
with raises(AssertionError):
    assert isinstance(obj, dict)  # got MyDataclass

Expected Behavior

None of the above assertions should raise an AssertionError; _convert_ should have the same effect for top-level configs as for nested configs.

System information

• Hydra Version : 1.4.0.dev0
• Python version : 3.10.8

Additional context

Based on bug report #2504.