[Inductor][bugFix] fix a bug in merge_splits (#119956)
Summary: RecGPT got a keyerror when running the split_cat, and it was caused by a corner case hit.
Test Plan: P1184947021
Differential Revision: D53791839
Pull Request resolved: https://github.com/pytorch/pytorch/pull/119956
Approved by: https://github.com/jackiexu1992
