[feature request] Allow visualization of gluon.nn.Sequential()

[ThomasDelteil]

I understand that visualizing gluon networks might be a bit trickier because of its dynamic nature. But I think that it would be nice to be able to recursively get the blocks that are part of Sequential network and display them. In my case I am using non-hybridizable blocks so can't get the -symbol.json

[reminisce]

@ThomasDelteil Thanks for trying this. I think it's not hard to only support Sequential non-hybridizable blocks, but the method is somewhat hacky since we have to assign unique names manually to each block, while in hybridizable nets this process is carried out automatically. We would like to come up with a more general, robust and backward compatibility guaranteed solution for Blocks.

I want to understand in your case why the network has to be Sequential instead of HybridSequential?

[ThomasDelteil]

Children of HybridBlock must also be HybridBlock
My network has a gluon.rnn.LSTM which is not hybrid