Implement luz_callback_validation_check

[mattwarkentin]

Related to https://github.com/mlverse/luz/issues/5#issuecomment-884426442

Hi @dfalbel,

This is a first attempt at implementing the validation check callback. It may still need some work so I am submitting this as a draft PR for now. By design, this check only runs a few batches and computes the loss. It does not strictly follow the standard validation loop because it does not call the validation-related callbacks.

We may also want to compute the validation metrics in this check. I will await your thoughts before more changes are made.