Add ephemeral merging label (#98543)
Addresses https://github.com/pytorch/test-infra/issues/3950
Test Plan: Ran a dry run on this pr. The label showed up while trying to merge
<img width="354" alt="Screenshot 2023-04-06 at 4 57 48 PM" src="https://user-images.githubusercontent.com/13758638/230514276-1ac70b58-d2d1-4e4b-892b-a957bf156063.png">
And then disappeared after failing
<img width="373" alt="Screenshot 2023-04-06 at 5 00 11 PM" src="https://user-images.githubusercontent.com/13758638/230514470-38b15ec7-cfd9-4efe-b6e8-0f9af5577c62.png">
There's also the trail of adding and removing the "merging" label at the bottom
Notes: This is slightly buggy sometimes. For example when the merge failed when I was editing this textbox, the label did not disappear.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/98543
Approved by: https://github.com/malfet
