[Minor] Improve the style and fix flaky tests (#1584)

python/sglang/srt/mem_cache/memory_pool.py

@@ -31,10 +31,13 @@ class ReqToTokenPool:
         self.size = size
         self.max_context_len = max_context_len
         self.device = device
-        self.free_slots = list(range(size))
         self.req_to_token = torch.empty(
             (size, max_context_len), dtype=torch.int32, device=device
         )
+        self.free_slots = list(range(size))
+
+    def available_size(self):
+        return len(self.free_slots)
 
     def alloc(self, need_size: int) -> List[int]:
         if need_size > len(self.free_slots):