fix cg acess issue by using dict instead of list to iteratively acces…

[ilml]

Fix nested CUDA-graph attribute access for Flex MoE token dispatchers.

Summary

• Use dotted-path helpers for token_dispatcher.cudagraph_attrs, so entries like _comm_manager.token_probs, _comm_manager.token_indices, and _comm_manager.routing_map are handled correctly.
• Replace flat getattr / setattr usage in MoE CUDA-graph capture and replay paths with shared nested-attribute helpers.
• Add unit tests covering nested cudagraph attribute reads and writes.

This keeps existing flat attribute handling unchanged while fixing Flex dispatcher backends such as deepep and hybridep.

What does this PR do ?

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[jiemingz]

https://github.com/NVIDIA/Megatron-LM/pull/3625/changes

There is an equivalent change on main, why not reflect this PR?

update: main has the exact code now except it misses the test, and doesnt refactor the setter and getter into their own functions. When we go to merge dev into main I can take an AI to make sure these get into main as well

[yaox12]

/ok to test bddcdb3

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[yaox12]

/ok to test 00d2b4b

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