add attention_sink.py

add KVCacheWithAttentionSink

Pull Request resolved: #6579

This PR adds `KVCacheWithAttentionSink`, which is required for `AttentionSink`. It keeps the first `sink_size` tokens as attention sinks and maintains a sliding window with `window_size` for new tokens.

Note: I am trying to implement and verify `AttentionSink` in eager mode first. So the current implementation may still have some lower errors. Will leave these problems to resolve when we are ready to deploy `AttentionSink` to edge.
ghstack-source-id: 255715047
@exported-using-ghexport

Differential Revision: [D65235798](https://our.internmc.facebook.com/intern/diff/D65235798/)

Co-authored-by: Lunwen He <[email protected]>

Author: pytorchbot

examples/models/llama/source_transformation/attention_sink.py

@@ -11,7 +11,7 @@
 
 import torch
 
-from executorch.examples.models.llama.llama_transformer import ModelArgs, Rope
+from executorch.examples.models.llama.llama_transformer import KVCache, ModelArgs, Rope
 from executorch.examples.models.llama.rope import (
     apply_rotary_emb_to_k,
     hf_apply_rotary_emb_to_k,
@@ -87,3 +87,122 @@ def rerotate_k(
         )
 
         return self.apply_rotary_emb_to_k(k, rerotation_cos, rerotation_sin)
+
+
+class KVCacheWithAttentionSink(KVCache):
+    """
+    KV cache that supports attention sink. It keeps the initial few tokens as attention sink.
+    For other tokens, it uses a sliding window to keep the most recent tokens.
+
+    Parameters:
+        window_size: the size of the sliding window
+        sink_size: the number of initial tokens to keep as attention sink
+        eviction_batch_size: the number of tokens to evict in batch when there is not enough space in the KV cache
+    """
+
+    def __init__(
+        self,
+        n_heads: int,
+        head_dim: int,
+        transpose_cache: bool,
+        enable_dynamic_shape: bool,
+        rope: RopeWithAttentionSink,
+        window_size: int,
+        sink_size: int,
+        eviction_batch_size: int,
+        max_batch_size: int = 1,
+        dtype=torch.float32,
+    ):
+        super().__init__(
+            max_batch_size=max_batch_size,
+            max_seq_length=window_size + sink_size,
+            n_heads=n_heads,
+            head_dim=head_dim,
+            transpose_cache=transpose_cache,
+            enable_dynamic_shape=enable_dynamic_shape,
+            dtype=dtype,
+        )
+        self.rope = rope
+        self.window_size = window_size
+        self.sink_size = sink_size
+        self.eviction_batch_size = eviction_batch_size
+        self.position_shift = 0
+
+    def evict_tokens(self, input_pos: torch.Tensor, seq_len: int) -> int:
+        """
+        Evict old tokens from the cache to make rooms for new tokens.
+
+        Parameters:
+            input_pos: the start position of the incoming token in the actual sequence
+            seq_len: the length of the incoming sequence
+            rope: the rope object to use for rerotating k
+
+        Returns:
+            the number of tokens to evict from the cache which is also the number of
+            positions to shift for incoming tokens
+        """
+        input_pos_item = input_pos.item()
+        torch._check_is_size(input_pos_item)
+        if input_pos_item + self.position_shift + seq_len > self.max_seq_length:
+            # There are not enough spaces in the cache to store the new tokens.
+            # We need to evict some old tokens and shift some recent tokens.
+            num_to_evict = max(
+                input_pos_item + self.position_shift - self.max_seq_length + seq_len,
+                self.eviction_batch_size,
+            )
+            num_to_keep = (
+                input_pos_item + self.position_shift - self.sink_size - num_to_evict
+            )
+            num_empty_space = self.window_size - num_to_keep
+            dim_to_slice = 2 if self.transpose_cache else 1
+            k_to_keep = self.k_cache.narrow(
+                dim_to_slice,
+                self.sink_size + num_to_evict,  # pyre-ignore [6]
+                num_to_keep,  # pyre-ignore [6]
+            )
+            if self.transpose_cache:
+                k_to_keep = self.rope.rerotate_k(
+                    k=k_to_keep.transpose(1, 2),
+                    original_position=(  # pyre-ignore [6]
+                        self.sink_size + num_to_evict
+                    ),
+                    new_position=self.sink_size,
+                ).transpose(1, 2)
+            else:
+                k_to_keep = self.rope.rerotate_k(
+                    k=k_to_keep,
+                    original_position=(  # pyre-ignore [6]
+                        self.sink_size + num_to_evict
+                    ),
+                    new_position=self.sink_size,
+                )
+            self.k_cache = torch.cat(
+                [
+                    self.k_cache.narrow(dim_to_slice, 0, self.sink_size),
+                    k_to_keep,
+                    torch.zeros_like(
+                        self.k_cache.narrow(
+                            dim_to_slice, 0, num_empty_space  # pyre-ignore [6]
+                        )
+                    ),
+                ],
+                dim=dim_to_slice,
+            )
+            self.v_cache = torch.cat(
+                [
+                    self.v_cache.narrow(dim_to_slice, 0, self.sink_size),
+                    self.v_cache.narrow(
+                        dim_to_slice,
+                        self.sink_size + num_to_evict,  # pyre-ignore [6]
+                        num_to_keep,  # pyre-ignore [6]
+                    ),
+                    torch.zeros_like(
+                        self.v_cache.narrow(
+                            dim_to_slice, 0, num_empty_space  # pyre-ignore [6]
+                        )
+                    ),
+                ],
+                dim=dim_to_slice,
+            )
+            self.position_shift -= num_to_evict  # pyre-ignore [8]
+        return self.position_shift