[Bugfix] Fix incorrect types in LayerwiseProfileResults (#12196)

Signed-off-by: Yuan Tang <terrytangyuan@gmail.com>

vllm/profiler/layerwise_profile.py

 import copy
 from collections import defaultdict
 from dataclasses import asdict, dataclass, field
-from typing import Callable, Dict, List, Optional, Tuple, TypeAlias, Union
+from typing import Any, Callable, Dict, List, Optional, Tuple, TypeAlias, Union
 
 import pandas as pd
 from torch._C._autograd import DeviceType, _KinetoEvent, _ProfilerResult
@@ -128,7 +128,7 @@ class LayerwiseProfileResults(profile):
         ])
         df.to_csv(filename)
 
-    def convert_stats_to_dict(self) -> str:
+    def convert_stats_to_dict(self) -> dict[str, Any]:
         return {
             "metadata": {
                 "num_running_seqs": self.num_running_seqs
@@ -227,7 +227,7 @@ class LayerwiseProfileResults(profile):
             [self._cumulative_cuda_time(root) for root in self._module_tree])
 
     def _build_stats_trees(self):
-        summary_dict: Dict[str, self.StatsTreeNode] = {}
+        summary_dict: Dict[str, _StatsTreeNode] = {}
         total_cuda_time = self._total_cuda_time()
 
         def pct_cuda_time(cuda_time_us):