Many operators don't have unit tests.

Our Python unit tests don't cover all operators. Recently, I found softmax activation was never tested and the test has been added in https://github.com/apache/incubator-mxnet/pull/10918. Currently, the backward computation of norm doesn't have GPU tests. I'm afraid more operators don't have unit tests.