DM-52004: Flatten input collections when a new output chain.

[MichelleGower]

Checklist

• ran Jenkins
• ran and inspected package-docs build
• added a release note for user-visible changes to doc/changes

[codecov]

Codecov Report

❌ Patch coverage is 0% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 86.35%. Comparing base (a207212) to head (4402b26).
⚠️ Report is 2 commits behind head on main.
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
...ython/lsst/pipe/base/script/transfer_from_graph.py	0.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #509      +/-   ##
==========================================
- Coverage   86.36%   86.35%   -0.02%     
==========================================
  Files         129      129              
  Lines       17068    17069       +1     
  Branches     2087     2087              
==========================================
- Hits        14741    14740       -1     
- Misses       1822     1824       +2     
  Partials      505      505              

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.