Update version.txt to 0.20.1a0

[kit1980]

No description provided.

[pytorch-bot]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/vision/8642

• 📄 Preview Python docs built from this PR

Note: Links to docs will display an error until the docs builds have been completed.

✅ You can merge normally! (5 Unrelated Failures)

As of commit 2ebc20e with merge base db5f8a0 ():

FLAKY - The following job failed but was likely due to flakiness present on trunk:

• Tests / unittests-windows (3.9, windows.g5.4xlarge.nvidia.gpu, cuda, 11.8) / windows-job (gh) (detected as infra flaky with no log or failing log classifier)

BROKEN TRUNK - The following jobs failed but was present on the merge base:

👉 Rebase onto the `viable/strict` branch to avoid these failures

• Build Windows Conda / pytorch/vision / upload / conda-py3_9-cpu (gh) (trunk failure)
• Build Windows Conda / pytorch/vision / upload / conda-py3_9-cuda11_8 (gh) (trunk failure)
• Build Windows Conda / pytorch/vision / upload / conda-py3_9-cuda12_1 (gh) (trunk failure)
• Build Windows Conda / pytorch/vision / upload / conda-py3_9-cuda12_4 (gh) (trunk failure)

This comment was automatically generated by Dr. CI and updates every 15 minutes.

[atalman]

I believe we don't need this since version is already 0.20.0