We are (well, AMD is) working on it. It's all being done in the open; there are PRs you can follow. You can even help out if you're so inclined.

It's not going to be simple or fast, but it's coming...

(I work on XLA, a compiler for TensorFlow, and I've been working closely with AMD on the TF/XLA -> AMDGPU port. My team also works on CUDA support in clang, and we've been reviewing AMD's patches to support HIP in upstream clang.)

There was a OpenCL branch no long ago that was supposed to make it into mainline at some point. You can (kinda) track progress of that here: https://github.com/tensorflow/tensorflow/issues/22