title: Efficient allreduce is not supported for 2 IndexedSlices
body: <p>I am trying to run a Subclassed Keras Model on multiple GPUs. The code is running as expected, however, the following "warning" crops up during the execution of the code: </p>
  
  <p>"Efficient allreduce is not supported for 2 IndexedSlices"</p>
  
  <p>What does this mean? I followed the Multi-GPU tutorial on Tensorflow 2.0 Beta guide. I am also using the Dataset API for my input pipeline.</p>