fix vid dataloader frame selection

[wr0124]

This dataloader generates a pool of available starting frame numbers for each video series and selects one from that pool. It works with:
python3 -W ignore::UserWarning train.py
--dataroot /path/to/data
--checkpoints_dir /path/to/checkpoints
--name test_debug
--gpu_ids 0
--data_relative_paths
--model_type palette
--data_dataset_mode self_supervised_vid_mask_online
--train_batch_size 1
--train_iter_size 1
--data_num_threads 1
--model_input_nc 3
--model_output_nc 3
--data_relative_paths
--train_G_ema
--train_optim adamw
--G_netG unet_vid
--data_online_creation_rand_mask_A
--train_G_lr 0.0002
--dataaug_no_rotate
--G_diff_n_timestep_train 8
--G_diff_n_timestep_test 3
--data_temporal_frame_step 1
--alg_diffusion_cond_image_creation computed_sketch
--alg_diffusion_cond_computed_sketch_list canny
--diffusion_cond_sketch_canny_range 0 150
--alg_diffusion_vid_canny_dropout 0.0 0.0
--output_print_freq 1
--output_display_freq 1
--data_temporal_number_frames 8
--data_online_creation_crop_size_A 32
--data_online_creation_crop_size_B 32
--data_crop_size 32
--data_load_size 32
--train_compute_metrics_test
--train_metrics_every 1
--train_metrics_list PSNR LPIPS SSIM \