Provide support for functools.partial within config-creation funcs

[rsokl]

Note that this issue is not describing support for creating a partial-config; we already have this capability:

>>> instantiate(builds(dict, a=1, zen_partial=True))
functools.partial(dict, a=1)

rather it is about accommodating user-supplied/default values and builds-targets that are themselves partial'd targets.

Supporting builds(functools.partial(<target>, ...), ...)

Currently, attempting to use builds on a "partial'd" target raises an error because the __name__ of the object cannot be resolved (which is needed to determine _target_):

from hydra_zen import builds

from functools import partial

>>> builds(partial(dict))
---------------------------------------------------------------------------
AttributeError 
AttributeError: functools.partial(<class 'dict'>) does not have a `__name__` attribute

We can actually support this by "unpacking" the partial'd object. Under the hood, builds can detect if target "looks like" a partial and do:

 return builds(target.func, *target.args, **target.kwargs)

so that we get:

>>> Conf = builds(partial(dict, a=2))
>>> Conf
types.Builds_dict
>>> instantiate(Conf)  # should produce same result as `partial(dict, a=2)()`
{'a': 2}

As an aside, we should improve our error-reporting in this case. The reported attribute error will likely not lead users to a solution.

Supporting builds(target, x=functools.partial(func, a=1))

We can also support cases where a configured field is itself a partial'd object. We can similarly "unpack" the partial, but this time we will apply zen_partial=True so that the config instantiates back to the partial'd object.

E.g.

Conf = make_config(x=functools.partial(dict, a=1))

will effectively be equivalent to

Conf = make_config(x=builds(dict, a=1, zen_partial=True))

so that

>>> obj = instantiate(make_config(x=functools.partial(dict, a=1)))
>>> obj.x
functools.partial(dict, a=1)