I always see these tags in the issues and pull requests on Github but do not know the meaning and find it hard to search for.
For example, in this famous Caffe project.
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These are custom labels. In the example you give, they seem to be the initials of developers.
Looking at the project's page, the lead developer is called Evan Shelhamer (ES). JD seems to be a contributor named Jeff Donahue. JL would be contributor Jon Long. All these people are mentioned on the project's page, under "The BVLC members who have contributed to Caffe are ..."
In other words, not just contributors, but people from the Berkeley Vision and Learning Centre (BVLC) itself - people with leader(-like) positions on the project.