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class Backbone(torch.nn.Module,  Registrable)

A Backbone operates on basic model inputs and produces some encoding of those inputs that will be shared among one or more Heads in a multi-task setting. For plain text inputs, this is often a transformer.

The main purpose of this class is to give us a Registrable class that we can use as a type annotation on Model classes that want to use a backbone. The expectation is that this will take the same inputs as a typical model, but return intermediate representations. These should generally be returned as a dictionary, from which the caller will have to pull out what they want and use as desired. As a convention that these modules should generally follow, their outputs should have the same name as the given input, prepended with encoded_. So, a backbone that encodes a text input should return an output called encoded_text. This convention allows easier exchangeability of these backbone modules.

Additionally, as downstream Heads will typically need mask information, but after encoding have no way of computing it, a Backbone should also return a mask for each of its outputs, with the same name as the output but with _mask appended. So in our example of text as input, the output should have an entry called encoded_text_mask.

Because a Backbone handles model inputs, if you want to make those inputs human readable (e.g., for displaying them in a demo), then it's typically only the Backbone object that knows how to do that. So we also implement the make_output_human_readable function from the Model class. The implementation in the base class does nothing, but concrete classes should generally convert whatever input indices are saved to the output into text.


class Backbone(torch.nn.Module,  Registrable):
 | ...
 | def forward(self, **kwargs) -> Dict[str, torch.Tensor]


class Backbone(torch.nn.Module,  Registrable):
 | ...
 | def make_output_human_readable(
 |     self,
 |     output_dict: Dict[str, torch.Tensor]
 | ) -> Dict[str, torch.Tensor]