Adding batch validation

[FadelMamar]

Hello Alexandre,

Thank you for the amazing work you've done with HerdNet.

Here are the changes:

• animaloc/datasets/folder.py: adding argument images_paths to override root_dir
• animaloc/eval/evaluators.py#L161 : inside the evaluate method, I have created a function batch_metric to compute metrics for batchsize>=1. Have a look at line 218 and 245 to see how it's beeing called and how to disable it.
• animaloc/eval/evaluators.py#L363 : changing the HerdNetEvaluator.prepare_feeding
• animaloc/data/batch_utils.py#L39: adding val_collate_fn to be used in the val/test dataloader

Looking forward to your feedbacks