You are storing the state_dict methods in:
torch.save(g_optimizer.state_dict, '/content/drive/MyDrive/GAN_Temp/g_optim.pth')
torch.save(d_optimizer.state_dict, '/content/drive/MyDrive/GAN_Temp/d_optim.pth')

and would need to call this method to store the actual state_dict:
torch.save(g_optimizer.state_dict(), '/content/drive/MyDrive/GAN_Temp/g_optim.pth')
torch.save(d_optimizer.state_dict(), '/content/drive/MyDrive/GAN_Temp/d_optim.pth')