Set names of output Variables from PickableSequentialChain

[ktns]

Set names of output Variables from PickableSequentialChain, so that they match with names in pick and layer_names.

This is for more readable computational graphs.

[yuyu2172]

There is a possibility that a user of PickableSequentialChain uses a function that outputs a variable with name...

[yuyu2172]

How about setting the name only when var.name is None?

[ktns]

If an instance of PicakbleSequentialChain has a nested PickableSequentialChain as its child, I think its better var.name is rewritten by the parent because the parent tends to have more information about the whole structure of the NN. I thought a scheme like parent_name/child_name is too complicated, but it might require reconsideration.