add: mask_correlated_samples_2 in AttnGAN+CL

[JoonHong-Kim]

I think the pretrain_DAMSM.py works reliably only when this function is also at masks.py in the AttnGAN+CL folder.

[huiyegit]

Maybe. My experiments for the pretrain_DAMSM.py were conducted under the AttnGAN+CL.

[JoonHong-Kim]

The mask_correlated_samples_2 function is used in AttnGAN+CL/code/pretrain_DAMSM.py, but it was not defined in the masks.py file in the same folder, so I modified it. In this case, DM-GAN+CL/code/masks.py outside file is imported, there is no problem functionally, but if the two folders are separated, there may be a problem.