Add a fix for single-GPU nsys.

[tfogal]

The SingleDeviceStrategy never sets the requisite attribute; workaround it in the profiling code.

What does this PR do ?

Fixes using the nsys callback for single-GPU benchmarking.

Collection: core

Changelog

• Fix for cuda profiling of single-GPU runs

Usage

   nsyscb = nl.pytorch.callbacks.nsys.NsysCallback(
     start_step=0,
     end_step=40,
     ranks=[0],
     gen_shape=False,
   )
  llm.api.finetune(..., callbacks=[nsyscb])

is how I am using it.

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• Related to # (issue)

[github-actions]

beep boop 🤖: 🙏 The following files have warnings. In case you are familiar with these, please try helping us to improve the code base.

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[akoumpa]

@tfogal NeMo commits need to have --signoff can you amend and push again? In the meantime I've started CI since it takes a few hours to complete. If CI has passed before you push we can ask someone to force merge if needed.