Add NVTX ranges to categorize execution

[minitu]

What does this PR do ?

Adds NVTX ranges inside NeMo to categorize different parts of the execution.
For now the PR only adds an NVTX range around the MCore optimizer step.

Collection: [Note which collection this PR will affect]

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# Add a code snippet demonstrating how to use this 

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[minitu]

@gautham-kollu

[github-actions]

beep boop 🤖: 🚨 The following files must be fixed before merge!

---

Your code was analyzed with PyLint. The following annotations have been identified:

************* Module nemo.utils.nvtx
nemo/utils/nvtx.py:26:0: C0116: Missing function or method docstring (missing-function-docstring)
nemo/utils/nvtx.py:31:0: C0116: Missing function or method docstring (missing-function-docstring)

-----------------------------------
Your code has been rated at 8.18/10

---

Mitigation guide:

• Add sensible and useful docstrings to functions and methods
• For trivial methods like getter/setters, consider adding # pylint: disable=C0116 inside the function itself
• To disable multiple functions/methods at once, put a # pylint: disable=C0116 before the first and a # pylint: enable=C0116 after the last.

By applying these rules, we reduce the occurance of this message in future.

Thank you for improving NeMo's documentation!

[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.

---

Your code was analyzed with PyLint. The following annotations have been identified:

************* Module nemo.core.optim.mcore_optim
nemo/core/optim/mcore_optim.py:49:4: C0116: Missing function or method docstring (missing-function-docstring)
nemo/core/optim/mcore_optim.py:52:4: C0116: Missing function or method docstring (missing-function-docstring)
nemo/core/optim/mcore_optim.py:55:4: C0116: Missing function or method docstring (missing-function-docstring)
nemo/core/optim/mcore_optim.py:58:4: C0116: Missing function or method docstring (missing-function-docstring)
nemo/core/optim/mcore_optim.py:94:4: C0116: Missing function or method docstring (missing-function-docstring)
nemo/core/optim/mcore_optim.py:97:4: C0116: Missing function or method docstring (missing-function-docstring)
nemo/core/optim/mcore_optim.py:114:4: C0116: Missing function or method docstring (missing-function-docstring)
nemo/core/optim/mcore_optim.py:117:4: C0116: Missing function or method docstring (missing-function-docstring)
nemo/core/optim/mcore_optim.py:20:4: W0611: Unused MegatronOptimizer imported from megatron.core.optimizer.optimizer (unused-import)

-----------------------------------
Your code has been rated at 8.27/10

---

Mitigation guide:

• Add sensible and useful docstrings to functions and methods
• For trivial methods like getter/setters, consider adding # pylint: disable=C0116 inside the function itself
• To disable multiple functions/methods at once, put a # pylint: disable=C0116 before the first and a # pylint: enable=C0116 after the last.

By applying these rules, we reduce the occurance of this message in future.

Thank you for improving NeMo's documentation!

[minitu]

@marcromeyn @hemildesai Pinging for reviews, code has been updated to use NsysCallback and AppState

[gautham-kollu]

LGTM