Unused imports cleanup
jenkins There are also many odd choices, many torch.nn imports have been removed and arbitrarily using the longer torch.nn.xyz - why ?  > There are also many odd choices, many torch.nn imports have been removed and arbitrarily using the longer torch.nn.xyz - why ?

The suggestions come from Flake8 tool, I just ran: `flake8 . | grep F401 | grep -v "_init__.py"`, manually reviewed and applied them. So these are in fact just unused imports.

The only case I changed something is type annotation here https://github.com/NVIDIA/NeMo/blob/a0456c6ce1ac0731c0cec2acd3047c6ecb72e75d/examples/nlp/language_modeling/megatron_change_num_partitions.py#L1220

I can revert that if you prefer.
 > This will create merge conflicts in practically every open pr. Why is this PR necessary ? If it's aesthetics, it's not sufficient reason to merge this.
> 
> @artbataev has the same issue with black, but has found a way to make it iterative and only pr level change where than global change that affects all open prs

* I think `git` should be able handle conflicts automatically, these are typically one-line changes. They are effectively no-ops, and Jenkins CI passes. Yes, that's mostly for aesthetics -- I discussed with Eric that it's worth it
* I would recommend looking into full Flake8 output besides, it discovers many issues besides (including undefined variables) so this is a low-hanging-fruit improvement The CI passes because it's testing this branch without any conflicts. Try manually rebasing a copy if any large open pr with your branch here and see if Pycharm can handle it automatically. Vlad and I have tried it, it could not handle blacks "one line changes" automatically either  If Pycharm can rebase and handle this PR with other large pr without requiring manual review, then it's fine. Otherwise it's not worth introducing merge conflicts across 80 open prs for an aesthetic change  This PR is stale because it has been open for 14 days with no activity. Remove stale label or comment or update or this will be closed in 7 days. This PR was closed because it has been inactive for 7 days since being marked as stale.
# What does this PR do ?

Unused imports cleanup indentified by [Flake8](https://flake8.pycqa.org/en/latest), see https://www.flake8rules.com/rules/F401.html.

**Collection**: [ALL]

# Jenkins CI
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# Additional Information
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