(torchx/specs) temporarily add back base_image to Role to keep backward compatibility with older fbpkgs in workflows (#229)

Summary:
Pull Request resolved: #229

makes torchx_run_gang workflow backward compatible with workflows that still have base_image

Solves: https://fb.workplace.com/groups/140700188041197/permalink/203080498469832/

Reviewed By: kiukchung

Differential Revision: D31513305

fbshipit-source-id: b72f47b072fa1971983fd5d1c2115242aadee9d7

Author: aivanou

torchx/specs/api.py

@@ -221,6 +221,7 @@ class Role:
 
     name: str
     image: str
+    base_image: Optional[str] = None  # DEPRECATED DO NOT SET, WILL BE REMOVED SOON
     entrypoint: str = MISSING
     args: List[str] = field(default_factory=list)
     env: Dict[str, str] = field(default_factory=dict)