use default_factory and not a mutable dictionary (#1788)

Summary:
Sorry, I broke this yesterday

Pull Request resolved: #1788

Reviewed By: xuzhao9

Differential Revision: D47810975

Pulled By: janeyx99

fbshipit-source-id: 9109d2446e4d41d917f957eb9925f757126f058b

userbenchmark/utils.py

@@ -6,7 +6,7 @@
 import json
 import yaml
 from pathlib import Path
-from dataclasses import dataclass
+from dataclasses import dataclass, field
 from typing import Any, Dict, List, Optional, Callable
 
 REPO_PATH = Path(os.path.abspath(__file__)).parent.parent
@@ -49,8 +49,8 @@ class TorchBenchABTestResult:
     control_env: Dict[str, str]
     treatment_env: Dict[str, str]
     details: Dict[str, TorchBenchABTestMetric]
-    control_only_metrics: Dict[str, float] = {}
-    treatment_only_metrics: Dict[str, float] = {}
+    control_only_metrics: Dict[str, float] = field(default_factory=dict)
+    treatment_only_metrics: Dict[str, float] = field(default_factory=dict)
     # the repository to bisect, default to "pytorch"
     bisection: str = "pytorch"
     # can be "abtest" or "bisect"