dblp seems the best i found so far (as also mentioned in the comments and in What is the best place to get BibTeX entries for computer science articles ?) but even there some papers from NIPS workshops and ICLR in general are not listed as such and are listed simply as CoRR papers.
For example "Neural Machine Translation by Jointly Learning to Align and Translate"
although after writing the question i'm finding that most papers are referencing the NMT paper also as CoRR/arXiv. I also didn't find proceedings of ICLR so maybe it's a general issue only with ICLR papers and some conference workshops.