Avoid crash on empty translation

app.py

@@ -92,11 +92,14 @@ with tokenizer.as_target_tokenizer():
         'cumulative probability': probs_for_likely_tokens.cumsum(0)
     })
 
+st.subheader("Most likely next tokens")
 st.write(probs_table)
 
-loss_table = pd.DataFrame({
-    'token': [tokenizer.decode(token_id) for token_id in decoder_input_ids[1:]],
-    'loss': F.cross_entropy(model_output.logits[0, :-1], torch.tensor(decoder_input_ids[1:]).to(device), reduction='none').cpu()
-})
-st.write(loss_table)
-st.write("Total loss so far:", loss_table.loss.sum())
+if len(decoder_input_ids) > 1:
+    st.subheader("Loss by token")
+    loss_table = pd.DataFrame({
+        'token': [tokenizer.decode(token_id) for token_id in decoder_input_ids[1:]],
+        'loss': F.cross_entropy(model_output.logits[0, :-1], torch.tensor(decoder_input_ids[1:]).to(device), reduction='none').cpu()
+    })
+    st.write(loss_table)
+    st.write("Total loss so far:", loss_table.loss.sum())