Improve readability of PixelShuffle operation in Llama4

[Yuki-Imajuku]

This PR improves the readability of the pixel_shuffle_op() function within llama_models/llama4/vision/embedding.py.

Previously, the input tensor dimensions were unpacked as n, w, h, c = input_x.size(), but since the tensor dimensions within PixelShuffle.forward() are expected to be $[N, H, W, C]$, this PR aligns the naming accordingly.

Since the implementation assumes a square patch input, this change does not affect the underlying logic and is strictly for readability enhancement.

Additionally, the type annotation for ps_ratio has been added because it differs slightly from torch.nn.PixelShuffle by accepting a float.

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