Add evaluation results on the default config of xsum

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Your model has been evaluated on the default config of the [xsum](https://huggingface.co/datasets/xsum) dataset by

@nbroad

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README.md

@@ -6,7 +6,40 @@ tags:
 - summarization
 model-index:
 - name: longt5-mediasum
-  results: []
+  results:
+  - task:
+      type: summarization
+      name: Summarization
+    dataset:
+      name: xsum
+      type: xsum
+      config: default
+      split: test
+    metrics:
+    - name: ROUGE-1
+      type: rouge
+      value: 22.7044
+      verified: true
+    - name: ROUGE-2
+      type: rouge
+      value: 5.616
+      verified: true
+    - name: ROUGE-L
+      type: rouge
+      value: 18.0111
+      verified: true
+    - name: ROUGE-LSUM
+      type: rouge
+      value: 18.1554
+      verified: true
+    - name: loss
+      type: loss
+      value: 2.1656227111816406
+      verified: true
+    - name: gen_len
+      type: gen_len
+      value: 18.3527
+      verified: true
 ---
 
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