Update the implementation of ssim() loss function, reduce the computational complexity from O(n) to O(1) since create_window() is called only once in main function

[tryhiseyyysum]

In the loss_utils.py: Updated the function ssim(), the optimized version of ssim() is ssim_optimized(), which reduce the computational complexity. After modified, the function create_window() just need to be called once in the main function in train.py, no need to be called in ssim() in every iteration.

In the train.py: Updated the main function, call the create_window() function for just once, since the window_size is fixed to 11 and the channel is fixed to 3 according to the source code. Updated the calculation of loss function, call the ssim_optimized(), instead of the original ssim().