[jit] Support named tuple return from operators

[ezyang]

#15429 makes the following syntax valid in PyTorch programs:

ret = a.svd()
U = ret.U  # previously, only ret[0] was supported

This syntax is not currently understood by TorchScript. It should be handled.

cc @zasdfgbnm (not saying you have to do this, but just to let you know :)

[zasdfgbnm]

@ezyang I am planning to do this, could you assign this issue to me?

[ezyang]

GH is being stupid and won't let me assign the issue to you. But your involvement has been duly noted :)