Configuring methods of classes (especially: wrapped by external_configurable)

Hey folks,

When I want to configure individual methods of a class, i.e. to write in `config.gin` something like:

```python
Foo.bar.x = 10
```

If i'm not mistaken we need two layers of configurable decorators on our class, like so:

```python
@gin.configurable
@dataclass
class Foo:
x: float = 5.

@gin.configurable(module="Foo")
def bar(self, y: float):
print(y)

@classmethod
@gin.configurable(module="Foo")
def baz(cls, xstr: str):
return cls(float(xstr))
```

This is fine, I guess the interpretation of the top-level decorator is that we only want to configure the __init__, and want to be explicit about which individual methods are configurable.

How do we tackle binding to method arguments on an `external_configurable`? In particular, I want to do something in `config.gin` like:

```
import llama_index
import mymodule.gin # inside I import ServiceContext and run gin.external_configurable on it

# ...
llama_index.ServiceContext.from_defaults.llm = %llm
```

Do I need to monkey patch gin-decorated versions of each method in the class from the external library?

Thanks for any advice or thoughts. Absolutely love this library btw.