bump version number from 1.2 to 1.3 after v1.2.0 release (#3054)

Summary:
Pull Request resolved: #3054

# context
* TorchRec release v1.2.0 to PYPI: https://pypi.org/project/torchrec/1.2.0/
* github release notes: https://github.com/pytorch/torchrec/releases
* version bump from `1.2.0a0` to `1.3.0a0`, which means very new feature will go to v1.3.0

Reviewed By: spmex

Differential Revision: D76153712

fbshipit-source-id: e1686c0f8285c578ccd536ce91b370758d60fccb

Author: TroyGarden

version.txt

@@ -1 +1 @@
-1.2.0a0
+1.3.0a0