Fix version number on main branch (#2509)

Summary:
The source build is still saying its 0.12.

Pull Request resolved: https://github.com/pytorch/audio/pull/2509

Reviewed By: carolineechen

Differential Revision: D37427703

Pulled By: mthrok

fbshipit-source-id: a6e455ba7c583af7b1a2a355ca45a9e5ab5fe30d

setup.py

@@ -22,7 +22,8 @@ def _run_cmd(cmd):
 
 
 def _get_version(sha):
-    version = "0.12.0a0"
+    with open(ROOT_DIR / "version.txt", "r") as f:
+        version = f.read().strip()
     if os.getenv("BUILD_VERSION"):
         version = os.getenv("BUILD_VERSION")
     elif sha is not None: