Add evaluation results on xsum

Beep boop, I am a bot from Hugging Face's automatic evaluation service! Your model has been evaluated on the [xsum](https://huggingface.co/datasets/xsum) dataset. Accept this pull request to see the results displayed on the [Hub leaderboard](https://huggingface.co/spaces/autoevaluate/leaderboards?dataset=xsum). Evaluate your model on more datasets [here](https://huggingface.co/spaces/autoevaluate/autoevaluate?dataset=xsum).

README.md

@@ -20,6 +20,31 @@ model-index:
     - name: Rouge1
       type: rouge
       value: 23.9405
+  - task:
+      name: Summarization
+      type: summarization
+    dataset:
+      type: xsum
+      name: xsum
+      config: default
+      split: test
+    metrics:
+    - name: ROUGE-1
+      type: rouge
+      value: 18.0808
+      verified: true
+    - name: ROUGE-2
+      type: rouge
+      value: 3.3971
+      verified: true
+    - name: ROUGE-L
+      type: rouge
+      value: 14.3604
+      verified: true
+    - name: ROUGE-LSUM
+      type: rouge
+      value: 14.4749
+      verified: true
 ---
 
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