Add hydra_defaults argument to builds

[rsokl]

Hydra supports specifying defaults on targeted configs:

from dataclasses import dataclass, field
from typing import Any

from hydra_zen import launch

import hydra
from hydra.core.config_store import ConfigStore


class MyClass:
    def __init__(self, **kwargs):
        self.kwargs = kwargs


Default = builds(int)

cs = ConfigStore.instance()
cs.store(group="x", name="a", node=Default)

@dataclass
class Conf:
    _target_: str = "__main__.MyClass"
    defaults: Any = field(default_factory=lambda: ["_self_", {"x": "a"}])
    x: Any = None
    
cs.store(name="my_app", node=Conf)  

@hydra.main(config_path=None, config_name="my_app")
def task_function(cfg):
    print(instantiate(cfg).kwargs)

Launching the task function sets the default and does not pass defaults to MyClass(...)

>>> launch(Conf, task_function);
{'x': 0}

Presently, builds can accommodate this via meta-field:

builds(MyClass, x=None, zen_meta=dict(defaults=["_self_", {"x": "a"}]))

But it might be nice to expose this as an explicit argument, hydra_defaults:

builds(MyClass, x=None, hydra_defaults=["_self_", {"x": "a"}])

The "pros" of adding hydra_defaults are:

• It is explicit
• It is a Hydra feature, and thus people might expect to see parity with hydra_convert and other hydra features.
• It does not require users to understand meta-fields
• The config-creation code and resulting yaml will be cleaner

The "cons" are:

• It might be quite rare that people specify defaults on a targeted config
• The meta-field workflow already works! Plus the meta-field is explicitly excluded, by hydra-zen, from all instantiation