Post release 2.19.0

[arcra]

Several commits cherry-picked from the release branch, plus a commit to bump the version at HEAD, for our tb-nightly releases, to round out the 2.19.0 release process.

For reference, the cherry-picks from the release branch addressed these couple of things:

• There was a "bug" introduced in Update instructions to sync TF compat protos #6916 , in which some curly braces that are meant to be printed literally are interpreted as place holders for string interpolation in python. Adding double curly braces makes them be "escaped" to be printed out.
• Some protobuffers have been updated in TF, and we updated our "compat" protos to work without TF. Notably, some of the source protos were moved to a different location in this commit and this PR.

Then the normal post-release commits are included:

• Bump version for tb-nightly to 2.20.0a0.
• Cherry-pick release notes.