Removed unnecessary self.projection call in VivitTubeletEmbeddings

[v-iashin]

What does this PR do?

Fixes #31619

self.projection is called twice here:

transformers/src/transformers/models/vivit/modeling_vivit.py

Lines 75 to 80 in 0f67ba1

	pixel_values = pixel_values.permute(0, 2, 1, 3, 4)
	x = self.projection(pixel_values)
	# out_batch_size, out_num_channels, out_num_frames, out_height, out_width = x.shape
	x = self.projection(pixel_values).flatten(2).transpose(1, 2)
	return x

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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• This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
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• Did you write any new necessary tests?

Who can review?

@amyeroberts

[amyeroberts]

Thanks for spotting and the quick fix!