[ExecuTorch] Some updated to kv cache

Update kv cache impl to consider untransposed cache

Differential Revision: [D62301843](https://our.internmc.facebook.com/intern/diff/D62301843/)

[ghstack-poisoned]

Author: kimishpatel

examples/models/llama2/llama_transformer.py

@@ -151,6 +151,7 @@ def __init__(
     ):
         super().__init__()
         self.max_seq_length = max_seq_length
+        self.is_tranposed = transpose_cache
         if transpose_cache:
             cache_shape = (max_batch_size, n_heads, max_seq_length, head_dim)
         else:
@@ -173,28 +174,34 @@ def update(
     ) -> Tuple[torch.Tensor, torch.Tensor]:
         # input_pos: [S], k_val: [B, H, S, D] or [B, S, H, D] depending on transpose_cache
         if self.enable_dynamic_shape:
-            start_pos = input_pos[-1].item()
+            start_pos = input_pos[0].item()
             torch._check_is_size(start_pos)
             torch._check(start_pos < self.max_seq_length)
-            seq_length = k_val.size(2)
+            dim_to_slice = 2 if self.transpose_cache else 1
+            seq_length = k_val.size(dim_to_slice)
             # Replace the entry in the cache for this token
             # The following lines are equivalent to:
             # cache_k[:bsz, start_pos : start_pos + seqlen] = xk
             # cache_v[:bsz, start_pos : start_pos + seqlen] = xv
+            # when dim_to_slice is 1
             # We use .narrow() here to make the compiler happy
             # pyre-ignore: Incompatible parameter type [6]
-            narrowed_k = self.k_cache.narrow(2, start_pos, seq_length)
+            narrowed_k = self.k_cache.narrow(dim_to_slice, start_pos, seq_length)
             # pyre-ignore: Incompatible parameter type [6]
-            narrowed_v = self.v_cache.narrow(2, start_pos, seq_length)
+            narrowed_v = self.v_cache.narrow(dim_to_slice, start_pos, seq_length)
 
             narrowed_k.copy_(k_val)
             narrowed_v.copy_(v_val)
             return self.k_cache, self.v_cache
         else:
             k_out = self.k_cache
             v_out = self.v_cache
-            k_out[:, :, input_pos] = k_val
-            v_out[:, :, input_pos] = v_val
+            if self.transpose_cache:
+                k_out[:, :, input_pos] = k_val
+                v_out[:, :, input_pos] = v_val
+            else:
+                k_out[:, input_pos] = k_val
+                v_out[:, input_pos] = v_val
 
             return k_out, v_out