What's the input to the decoder part of a sequence to sequence autoencoder? I've seen certain examples of such an autoencoder (using LSTM's more often than not) but am still unclear.
For example, here in this often-cited paper by Dai & Le ('Semi Supervised Sequence Learning'), we have the following diagram:
What's the input to the decoder portion of the autoencoder here? In this example it's 'W-X-Y-Z.' But in general, is it the same as the input to the encoder? Or is it using the output from the previous timestep/LSTM cell as input?
Similarly, in another popular paper by Srivastava et. al ('Unsupervised Learning of Video Representations using LSTMs'), they have the following diagram:
It seems they're using the reversed input from the encoder as input here. However, there's a section as follows:
The decoder can be of two kinds – conditional or unconditioned. A conditional decoder receives the last generated output frame as input, i.e., the dotted input in Fig. 2 is present. An unconditioned decoder does not receive that input.
In the unconditioned decoder, what input does the decoder receive?
Thank you! I'm new here, so please be kind :)