Bump version to 2.1 (#3181)

Summary: Pull Request resolved: https://github.com/pytorch/audio/pull/3181

Reviewed By: nateanl

Differential Revision: D44167788

Pulled By: mthrok

fbshipit-source-id: 375293df836456adc40020d323efbc0aebc60d83

version.txt

-2.0.0a0
+2.1.0a0