fix export dynamic shapes key error (#2656)

Summary:
Pull Request resolved: https://github.com/pytorch/executorch/pull/2656

Patch to fix https://www.internalfb.com/intern/testinfra/diagnostics/281475337620056.281475110585361.1711391810/ Not sure why this error is being thrown. Using a dict should be fine and passes in other places. switching to list is also fine here so doing that to unblock ci in the meantime

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Reviewed By: tarun292

Differential Revision: D55331712

fbshipit-source-id: 37b874f53acd2bf73672cc5ea7214c46322473d0