fix: Fix "AttributeError: 'MHATokenToKVPoolHost' object has no attribute 'start_layer'"

[chus-chus]

Motivation

Fix issue #6005 (AttributeError: 'MHATokenToKVPoolHost' object has no attribute 'start_layer' when using --enable-hierarchical-cache).

Modifications

Replaced instances of self.start_layer with self.device_pool.start_layer in both MLA and MHA TokenToKVPoolHost classes (memory_controller.py).

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[xiezhq-hermann]

To merge with #6016.