Passing callable as instantiate _target_ fails for nested classes

[Jasha10]

Hydra's instantiate function allows users to pass a callable (rather than a
string) via the _target_ keyword argument. For example:

def fn():
    return "fn return"


class A:
    def __init__(self):
        print("in A.__init__")

    @classmethod
    def method(self):
        return "A.method return"

# passing a callable (rather than a string) as _target_:
fn_return = instantiate({}, _target_=fn)
A_instance = instantiate({}, _target_=A)
A_method_return = instantiate({}, _target_=A.method)

However, this fails when a nested class is passed as the _target_:

class A:
    class B:
        def __init__(self):
            print("in A.B.__init__")

A_B_instance = instantiate({}, _target_=A.B)  # ImportError: Encountered AttributeError while loading '__main__.B': module '__main__' has no attribute 'B'

The error message indicates that __main__.B could not be found. This indicates a mistake in handling the keyword argument _target_=A.B: it is __main__.A.B that should have been looked up, not __main__.B.