Fix return type annotation for _get_sentence_embedding to match dict return

[Copilot]

The _get_sentence_embedding method returns a dictionary with sentence_embedding and label_attention_matrix keys, but was annotated as returning torch.Tensor.

Changes

• Updated return type annotation from torch.Tensor to Dict[str, Optional[torch.Tensor]]
• Updated docstring to document dictionary structure and keys
• Fixed early returns in "first" and "last" aggregation methods to return dict instead of bare tensors
• Normalized type annotations to use Dict consistently (was mixing dict and Dict)

# Before
def _get_sentence_embedding(...) -> torch.Tensor:
    """Returns: torch.Tensor: Sentence embeddings"""
    if aggregation_method == "first":
        return token_embeddings[:, 0, :]  # Returns tensor directly
    # ...
    return {"sentence_embedding": ..., "label_attention_matrix": ...}

# After
def _get_sentence_embedding(...) -> Dict[str, Optional[torch.Tensor]]:
    """Returns: Dict containing 'sentence_embedding' and 'label_attention_matrix'"""
    if aggregation_method == "first":
        return {"sentence_embedding": token_embeddings[:, 0, :], "label_attention_matrix": None}
    # ...
    return {"sentence_embedding": ..., "label_attention_matrix": ...}

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