Several operators failed when change backward unit tests framework

[reyoung]

After PR #5027, several op tests are failed. They are:

• test_cross_entropy_op
• test_softmax_with_cross_entropy_op
• test_gru_unit_op

[QiJune]

Since we divide the running of the neural network into compile-time and run-time, we must set correct variable type and shape in compile-time. At the run-time, the ProgramDesc is passed to Executor. The LoDTensor/SelectedRows in Variable will be created in before Executor Running. At run-time infershape and compute, we will only get LoDTensor/SelectedRows, and not create them.

I made these compile-time infer variable type and infer shape in #5093.

NetOp will be removed soon. Follow unit tests depends on NetOp need to be rewritten:

• test_cond_py
• test_recurrent_py
• test_dynamic_recurrent_py