Handle y_true as class targets for GradCAM

[RaphaelMeudec]

This PR makes it possible for the user to pass y_true instead of just giving a class_index. It's helpful as users won't need to build their dataset with one class only, and will just pass the dataset they use for training

What's left to do:

• [ ] Fix tests
• [ ] Extend the use of y_true to all current methods
• [ ] Impact documentation