Add logprobs to results in service (pytorch#1628)

Summary:
Pull Request resolved: pytorch#1628

Logprobs are now added to results which can help better understand attribution scores.

Reviewed By: vivekmig

Differential Revision: D78491492

fbshipit-source-id: d647e859ac0c421a131a2e31a3c2e13c4292c41f

Author: craymichael

captum/attr/_core/llm_attr.py

@@ -55,6 +55,7 @@ class LLMAttributionResult:
     token_attr: Optional[Tensor]
     input_tokens: List[str]
     output_tokens: List[str]
+    output_probs: Optional[Tensor] = None
 
     @property
     def seq_attr_dict(self) -> Dict[str, float]: