Guard cudagraph input copy on whether data pointers have actually changed

[mathemakitten]

What does this PR do ?

It is not always guaranteed that when A.data_ptr() == B.data_ptr(), the ._copy() will be a no-op. We guard this accordingly to avoid overhead.

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[svcnvidia-nemo-ci]

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[svcnvidia-nemo-ci]

🔄 Merge queue validation started!

You can track the progress here: https://github.com/NVIDIA/Megatron-LM/actions/runs/23309218874

[svcnvidia-nemo-ci]

🔄 Merge queue validation started!

You can track the progress here: https://github.com/NVIDIA/Megatron-LM/actions/runs/23312781417