Communication: Extend(Extend(x))

if on different dimensions we can still improve the performance here by padding the innermost one the requisite number of times.

The issue provides no file, test, or entry point. Start by locating the implementation and tests for Extend and its handling of multiple dimensions, then determine how performance is measured; done means improving the different-dimension case by padding the innermost dimension as described.

Written by the indexing model from the issue text.