Apparently, they thought of that, Line 3 of the patent application US10452978B2:
Provisional Application No. 62/510,256, filed on May 23, 2017
There are however other patents and applications:
Neural machine translation with latent tree attention
US20180300317A1 James BRADBURY
I'm not an expert so I can't read the spec and claims as to relevance with authority. However, it still wouldn't count directly as prior art as it was published in Oct 2018. Patents are now first to file and not first to invent (to match the rest of the world).
edit: They do also site non-patent prior are regarding attention. Whether this covers self attention I'm not clear and of course their claims have been reviewed by the examiner in light of the art so they're presumed valid until re-exam.
Luong et al. "Effective approaches to attention based neural machine translation," arXiv 1508.04025v2, Sep. 20, 2015, 11 pages.Luong et al. "Effective approaches to attention based neural machine translation," arXiv 1508.04025v2, Sep. 20, 2015, 11 pages.