examples/research_projects/rag-end2end-retriever/lightning_base.py/CheckParamCallback/on_after_backward
class CheckParamCallback:    def on_after_backward(self, trainer, pl_module):
        # print(pl_module.model.rag)
        for name, param in pl_module.model.rag.named_parameters():
            if param.grad is None:
                print(name)