test(tailor): add test for name is none

[bwanglzu]

1. add tailor tests when user didn't specify a embedding_layer_name, we use the last layer as the feature extractor.
2. fix an issue in pytorch where last layer is the model itself. For instance, last layer of vgg16 has 2 dimensional outputs, so it was added into the embedding_layers. If user didn't specify an embedding_layer_name, the entire vgg model will be replaced with Identity.

note: after previous refactoring #82 , keras 's tailor behaviour started to different from paddle and torch. When use last layer as embedding_layer, keras will take the last layer while paddle and torch will replace the last layer is Identity.