Fix validation data in tutorials using MedNIST

[Warvito]

In some tutorials using MedNIST contains a bug regarding the validation set. For example, in

GenerativeModels/tutorials/generative/2d_vqvae/2d_vqvae_tutorial.py

Line 102 in 589d79e

val_datalist = [{"image": item["image"]} for item in train_data.data if item["class_name"] == "HeadCT"]

the validation set is composed by items from train_data.data instead val_data.data.

Same occurs in

GenerativeModels/tutorials/generative/2d_vqgan/2d_vqgan_tutorial.py

Line 127 in 4849c6b

val_datalist = [{"image": item["image"]} for item in train_data.data if item["class_name"] == "HeadCT"]

GenerativeModels/tutorials/generative/2d_vqvae_transformer/2d_vqvae_transformer_tutorial.py

Line 126 in 4d9f3ea

val_datalist = [{"image": item["image"]} for item in train_data.data if item["class_name"] == "HeadCT"]

GenerativeModels/tutorials/generative/2d_super_resolution/2d_stable_diffusion_v2_super_resolution.py

Line 142 in 4849c6b

val_datalist = [{"image": item["image"]} for item in train_data.data if item["class_name"] == "HeadCT"]

[Warvito]

GenerativeModels/tutorials/generative/2d_autoencoderkl/2d_autoencoderkl_tutorial.py

Line 104 in 4849c6b

val_datalist = [{"image": item["image"]} for item in train_data.data if item["class_name"] == "Hand"]

GenerativeModels/tutorials/generative/2d_ldm/2d_ldm_tutorial.py

Line 107 in 4d9f3ea

val_datalist = [{"image": item["image"]} for item in train_data.data if item["class_name"] == "Hand"]

GenerativeModels/tutorials/generative/2d_ddpm/2d_ddpm_compare_schedulers.py

Line 134 in f9106b8

val_datalist = [{"image": item["image"]} for item in train_data.data if item["class_name"] == "Hand"]