fix bug when calling _concat_caches in kv.py

[RixinLiu]

Description

Summary: (fix a bug when calling _concat_caches function)

BUG Description:

Traceback (most recent call last):
  File "/root/Misc/test.py", line 24, in <module>
    merged_cache = mem.get_cache([cache_item_1.id, cache_item_2.id])
                   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/root/miniconda3/lib/python3.12/site-packages/memos/memories/activation/kv.py", line 81, in get_cache
    return self._concat_caches(caches_to_merge)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/root/miniconda3/lib/python3.12/site-packages/memos/memories/activation/kv.py", line 245, in _concat_caches
    merged.key_cache.append(torch.cat(keys, dim=-2))
    ^^^^^^^^^^^^^^^^^^^^^^^
AttributeError: 'KeyValuesWrapper' object has no attribute 'append'

Solution:
The solution replaced merged.key_cache.append and merged.value_cache.append with operations on merged.layers[layer].keys and merged.layers[layer].values. This aligns with the DynamicCache structure, where keys and values are stored per layer. Additionally, the code now correctly retrieves keys and values from c.layers[layer].keys and c.layers[layer].values respectively.

Fix: #(not applicable)

Docs Issue/PR: (not applicable)

Reviewer: @(not applicable)

Checklist:

• I have performed a self-review of my own code | 我已自行检查了自己的代码
• I have commented my code in hard-to-understand areas | 我已在难以理解的地方对代码进行了注释
• I have added tests that prove my fix is effective or that my feature works | 我已添加测试以证明我的修复有效或功能正常
• I have created related documentation issue/PR in MemOS-Docs (if applicable) | 我已在 MemOS-Docs 中创建了相关的文档 issue/PR（如果适用）
• I have linked the issue to this PR (if applicable) | 我已将 issue 链接到此 PR（如果适用）
• I have mentioned the person who will review this PR | 我已提及将审查此 PR 的人