Don't require notebooks to have all imports at top

[jackgerrits]

Why are these changes needed?

Related issue number

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 ✅

Project coverage is 36.96%. Comparing base (b872789) to head (f7a6b4d).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1803   +/-   ##
=======================================
  Coverage   36.96%   36.96%           
=======================================
  Files          62       62           
  Lines        6528     6528           
  Branches     1447     1447           
=======================================
  Hits         2413     2413           
  Misses       3921     3921           
  Partials      194      194           

Flag	Coverage Δ
unittests	36.96% <ø> (ø)

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[BeibinLi]

LGTM. Like it.