fix: clean up hash_to_block_id mapping when deallocating blocks

[ggboooy]

Problem
In the BlockManager._deallocate_block method, when deallocating a block, the corresponding entry in the hash_to_block_id dictionary was not being removed. This causes the hash mapping of deallocated blocks to remain in the dictionary, leading to potential issues:
Memory leak: invalid mappings accumulate in the dictionary over time
Logic error: may incorrectly reference deallocated blocks

Solution
Add hash mapping cleanup logic in the _deallocate_block method:
Check if the block being deallocated has a valid hash value (hash != -1)
If so, remove the corresponding mapping from the hash_to_block_id dictionary

Changes
File: nanovllm/engine/block_manager.py
Method: _deallocate_block
Added: hash mapping cleanup logic using pop() with safe default

[ggboooy]

Good catch!

[pangkun248]

If the above changes take effect, can the if branch in the code be simplified directly?

if block_id in self.used_block_ids:
      block = self.blocks[block_id]
      block.ref_count += 1
else:
    block = self._allocate_block(block_id)

change to

block = self.blocks[block_id]
block.ref_count += 1