Unexpected error from espfit/tests/test_app_train_sampler.py

RuntimeError: Trying to backward through the graph a second time (or directly access saved tensors after they have >already been freed). Saved intermediate values of the graph are freed when you call .backward() or autograd.grad(). >Specify retain_graph=True if you need to backward through the graph a second time or if you need to access saved >tensors after calling backward.

This is not critical for current experiment purpose so force merge. Above error will be resolved in the upcoming PR.