[KVCache] Fix the reference counter in sequence fork (#16666)

This PR fixes a sequence reference counter bug in the KV cache:
when forking a child sequnece from an existing parent sequence,
the reference counter of hte parent sequence was not increased.
This leads to error when the child sequence is removed, where we
will check the parent's reference counter and find it is 0 and is
never changed unexpectedly.

Meanwhile, this PR updates the PagedKVCache tests with some latest
changes, including target-aware tile size selection.

Author: MasterJH5574

src/runtime/relax_vm/paged_kv_cache.cc

@@ -475,6 +475,7 @@ class PagedAttentionKVCacheObj : public AttentionKVCacheObj {
         << "Attention merge-score function not available. ForkSequence is thereby not supported.";
 
     int32_t parent_block_idx = parent_it->second.last_block_idx;
+    ++global_block_pool_[parent_block_idx].external_ref_cnt;
     // Create a child block with the parent block pointer.
     int32_t child_block_idx = GetFreeBlock();
     global_block_pool_[child_block_idx].start_pos = parent_it->second.seq_length;