Fix issue 1440 by applying new function registration decorator

[thinkall]

Why are these changes needed?

Fix #1440 by applying new function registration decorator.

@xianzhisheng , @LUK3ARK could you help verify if this PR fixes your user cases? Thanks.

Also fixed some usage of cache in notebooks and tests.

Related issue number

Closes #1440

Checks

• I've included any doc changes needed for https://microsoft.github.io/autogen/. See https://microsoft.github.io/autogen/docs/Contribute#documentation to build and test documentation locally.
• I've added tests (if relevant) corresponding to the changes introduced in this PR.
• I've made sure all auto checks have passed.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (7ceee5d) 39.62% compared to head (2b9a930) 50.93%.

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #1661       +/-   ##
===========================================
+ Coverage   39.62%   50.93%   +11.30%     
===========================================
  Files          57       57               
  Lines        6006     6006               
  Branches     1338     1456      +118     
===========================================
+ Hits         2380     3059      +679     
+ Misses       3433     2702      -731     
- Partials      193      245       +52     

Flag	Coverage Δ
unittests	50.84% <ø> (+11.22%)	⬆️

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