Saves luz objects to diskSource:
Allows saving luz fitted models to the disk. Objects can be loaded back with
an object of class 'luz_module_fitted' as returned by
path in file system so save the object.
Objects are saved as plain
.rds files but
obj$model is serialized
torch_save before saving it.
The ctx is naively serialized. Ie, we only use
saveRDS() to serialize it.
luz_save to work correctly if you have unserializable objects
in the ctx like
torch_tensors and external pointers in general.