fix window selection

[marcosolime]

The way you pick valid windows does not seem correct. In your np.random.choice, you are providing as input the length of boolean mask (projs_window_valid_indx.shape[0]), corresponding to valid and invalid windows, but in this way you may pick also False windows.

Instead, you want to return the indices of valid windows only (torch.where). Then, you generate a random permutation of the valid indices and select the first self.window_num.