Hey Sachin,
It's true that the current support for ONNX is mostly for feed forward models. Any kind of recurrent networks or loop like diffusion is hard to implement with only native node.
I've done some work to experiment with diffusion using GafferML but some of it had to be done with Expression nodes running some python for the loop mechanics in particular but still using ONNX for the heavy lifting with VAE, text encoder and UNet.
We discussed with John and Eric some ideas to allow for this kind of architectures in full native ONNX by exposing the graph within Gaffer so please let us know about your use case.
Hope this helps!
Cheers,
Lucien