fix(checkpoint): prevent arbitrary code execution via `_instantiator` in `load_from_checkpoint` (#21832)
Prevent arbitrary code execution via _instantiator in load_from_checkpoint
Restrict the checkpoint _instantiator hyperparameter to an allowlist of
trusted instantiators. Previously any import path stored in a checkpoint
was imported and called, allowing arbitrary code execution even under
torch.load(weights_only=True). Fixes #21822.