Refactor FourierEmb for optimization and clarity

[zuazo]

This pull request refactors the FourierEmb class by moving constant calculations from the forward method to the constructor. With this change, we're avoiding unnecessary computations every time the forward method is called. This also improves code clarity.

Changes:

• Moved the calculation of width, freqs_y, freqs_x, p_x, and p_y to the constructor.
• Updated the forward method to utilize pre-computed values, ensuring they are on the correct device.
• Removed duplicated line: *O, D = positions.shape.

I hope you'll find this change beneficial. I'm open to feedback and further modifications if necessary. And thank you for sharing all your research work and code!

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