TOML-like setting groups/sections [Enchancement]

Right now adding and removing new config values is quite tedious as we need to repeat the class/function name for each field:
```
MyClass.batch_size = 100
MyClass.num_epochs = 100
...
```

If the `gin` format resemble TOML so much, it would seem natural to introduce the section headers, i.e.:
```
[MyClass]
batch_size = 100
num_epochs = 100
...
```