mxnet_backend.clear_session should clear the placeholder name dictionary

[Neiko2002]

Hello together,

right now I try to use the Keras (MXNet) library as a model converter. If you can port an existing MXNet model to a Keras model then you can convert from this intermediate format to the Theano, CNTK and TensorFlow format. This also works the other way around.

To make this happen I need access to the names of all nodes. Unfortunately the input variable name has an appended number which increases all the time. There is no way to reset it.

It would be helpful if mxnet_backend.clear_session() would reset the placeholder_name_dict variable.