Removed unnecessary `self.projection` call in `VivitTubeletEmbeddings`
Author: v-iashin
Pull request: yes

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Fixes #31619

`self.projection` is called twice here:
https://github.com/huggingface/transformers/blob/0f67ba1d741d65b07d549daf4ee157609ce4f9c1/src/transformers/models/vivit/modeling_vivit.py#L75-L80

This PR fixes it and adds a comment about the output shape. As a result, it should make the forward pass of `ViViT` faster.

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