Add identity layer

[zhoonit]

Now we are in need of identity layer(do nothing flow everything).

This is specifically to solve the issue addressed here https://github.com/nnstreamer/nntrainer/commit/e76a186c8b4a7bea1d8d30ffd9f208cfd2ec78e8#r62007856

A -> outputs ->(a0, a1)
B -> outputs ->(b0, b1)

C -> concat(a0, b0) -> c0
D -> concat(a1, b1) -> d0

But we want C and D to have the same identifier.

So, we need identity layer which does not do much but wrap multiple tensors in to one identity connection. eg) for above example it has to be

A -> outputs ->(a0, a1)
B -> outputs ->(b0, b1)

C -> concat(a0, b0) -> c0
D -> concat(a1, b1) -> d0

E -> identity(c0, d0) -> (e0, e1)

Now user can access c0 and d0 with the same name.

[taos-ci]

:octocat: cibot: Thank you for posting issue #1793. The person in charge will reply soon.