fix(server): fix flash causal (#218)

server/text_generation_server/models/flash_causal_lm.py

@@ -453,7 +453,10 @@ class FlashCausalLM(Model):
                 )
             # Set in batch in case it needs to be used later in concatenate()
             batch.past_pad = self.past_pad
-            if len(batch) != 1:
+            if len(batch) == 1:
+                # present is already pre-padded
+                batch.past_key_values = present
+            else:
                 # Add padding after each sequence
                 # This will have the correct shape after the final past_key_values concatenation before the model
                 # forward