fix Vivit for video classification example
Author: Geometrein
Pull request: yes

# What does this PR do?
Both Vivit for video classification example implementations were not working.
Some of the issues included:
- Missing or unnecessary library Imports
- Multiple variables referenced before assignment.
- Missing docstring for `sample_frame_indices()` method
- Logical error in `sample_frame_indices()` method that would cause Numpy ValueError.
- Missing or unnecessary lines of code.
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## Before submitting
- [x] This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
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## Who can review?

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Models:

- text models: @ArthurZucker and @younesbelkada
- vision models: @amyeroberts
- speech models: @sanchit-gandhi
- graph models: @clefourrier

Library:

- flax: @sanchit-gandhi
- generate: @gante
- pipelines: @Narsil
- tensorflow: @gante and @Rocketknight1
- tokenizers: @ArthurZucker
- trainer: @sgugger

Integrations:

- deepspeed: HF Trainer/Accelerate: @pacman100
- ray/raytune: @richardliaw, @amogkam

Documentation: @sgugger, @stevhliu and @MKhalusova

HF projects:

- accelerate: [different repo](https://github.com/huggingface/accelerate)
- datasets: [different repo](https://github.com/huggingface/datasets)
- diffusers: [different repo](https://github.com/huggingface/diffusers)
- rust tokenizers: [different repo](https://github.com/huggingface/tokenizers)

Maintained examples (not research project or legacy):

- Flax: @sanchit-gandhi
- PyTorch: @sgugger
- TensorFlow: @Rocketknight1

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Comments:
cc @amyeroberts 

The docs for this PR live [here](https://moon-ci-docs.huggingface.co/docs/transformers/pr_25568). All of your documentation changes will be reflected on that endpoint.