Correctly build models and import call_context for older TF versions
Our import for `call_context` was wrong for older TF versions - this unfortunately makes it quite hard to load models on older TF versions! This PR fixes it, and sorry about the issue!

Fixes #24133
@amyeroberts The code is in a conditional block that checks TF versions. I'm currently testing all TF versions since 2.4 to make sure this works for all of them - give me a few minutes to finish that before we merge the PR!<|||||>Version testing looks good!<|||||>_The documentation is not available anymore as the PR was closed or merged._<|||||>Merging this now and will begin discussions about a patch release