Resolve Pyre FixMe's in basic_models.py (#1618)

Summary:
Pull Request resolved: #1618

This diff resolves pyre-fixme's in the `basic_models.py` file.

2 pyre-fixmes are not resolved since no need to explicitly handling.

1 .
```
# pyre-fixme[61]: `another_embedding` is undefined, or not always defined.
return embedding if another_input is None else embedding + another_embedding
```
2 .
```
# pyre-fixme [29]: `typing.Type[Future]` is not a function
result = Future()
```

Reviewed By: cyrjano

Differential Revision: D77327906

fbshipit-source-id: fd0c7748ef5e33c3eb5ad2b3d2eb4c65cf1e4644

Author: jimshao1999