Fix mistral with length > window_size for long prefills (rotary doesn't create long enough cos, sin). (#1571)

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Author: Narsil

server/text_generation_server/models/flash_mistral.py

@@ -460,7 +460,10 @@ def forward(self, batch: FlashMistralBatch) -> Tuple[torch.Tensor, torch.Tensor]
             max_s = batch.max_seqlen
             lm_head_indices = batch.prefill_head_indices
 
-        if self.model.max_past is not None:
+        if cu_seqlen_prefill is None and self.model.max_past is not None:
+            # In decode, not prefill, we're actually overwriting the KV-cache
+            # in a circular buffer mode.
+            # This makes sure the max_s for the decode pass is correct.
             max_s = min(self.model.max_past, max_s)
 
         bs = input_ids.shape[0]