Fix typing in Model.generate_token (#733)

## What does this PR do?

This PR fixes a minor type annotation issue in the signature of
`Model.generate_token`.

All existing overrides of `Model.generate_token` return
`Tuple[List[Generation], Optional[B]]`:

https://github.com/huggingface/text-generation-inference/blob/3ef5ffbc6400370ff2e1546550a6bad3ac61b079/server/text_generation_server/models/causal_lm.py#L535-L537

https://github.com/huggingface/text-generation-inference/blob/3ef5ffbc6400370ff2e1546550a6bad3ac61b079/server/text_generation_server/models/flash_causal_lm.py#L802-L804

https://github.com/huggingface/text-generation-inference/blob/3ef5ffbc6400370ff2e1546550a6bad3ac61b079/server/text_generation_server/models/seq2seq_lm.py#L589-L591

I suspect that back in 017a2a8 when `GeneratedText` and `Generation`
were separated, the function signature was not updated.

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      Pull Request section?
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CC @OlivierDehaene

server/text_generation_server/models/model.py

@@ -5,7 +5,7 @@
 from typing import List, Tuple, Optional, TypeVar, Type
 from transformers import PreTrainedTokenizerBase, PretrainedConfig
 
-from text_generation_server.models.types import Batch, GeneratedText
+from text_generation_server.models.types import Batch, Generation
 from text_generation_server.pb.generate_pb2 import InfoResponse
 
 B = TypeVar("B", bound=Batch)
@@ -52,7 +52,7 @@ def batch_type(self) -> Type[B]:
         raise NotImplementedError
 
     @abstractmethod
-    def generate_token(self, batch: B) -> Tuple[List[GeneratedText], Optional[B]]:
+    def generate_token(self, batch: B) -> Tuple[List[Generation], Optional[B]]:
         raise NotImplementedError
 
     def warmup(self, batch: B) -> Optional[int]: