Fix `BayesianDetectorModel.from_pretrained()` by calling `post_init()` (#48254)
Fix BayesianDetectorModel.from_pretrained by calling post_init()
BayesianDetectorModel.__init__ never called self.post_init(), so
all_tied_weights_keys -- assigned in PreTrainedModel.tie_weights() --
was never created. _move_missing_keys_from_meta_to_device dereferences
it unconditionally, so save_pretrained() worked but from_pretrained()
always raised AttributeError.
Fixes #48253