message: Add experimental field to XrtComputationConfig to pass metadata from
  the XRT compiler to the standard TensorFlow TPU execution op.
diff: @@ -49,6 +49,33 @@ message XLAComputationConfig {
  DeviceAssignment device_assignment = 6;
  // The debugging options to be passed to the XLA compilation process.
  xla.DebugOptions debug_options = 7;
  +
  + // Everything inside Experimental is subject to change and is not subject
  + // to API stability guarantees in
  + // https://www.tensorflow.org/guide/version_compat.
  + message Experimental {
  + message UpdateIndexPair {
  + int32 index = 1;
  + bool updated = 2;
  + }
  +
  + // stateful_input_indices is only useful when using XRT-compiled
  + // programs together with standard TensorFlow TPU execution ops, so should
  + // be ignored by most clients.
  + //
  + // Optionally the client can pass information about which inputs
  + // to the computation are updates to "stateful" quantities. Each
  + // element of stateful_input_indices includes an index indicating
  + // which input argument it corresponds to, and a bool indicating
  + // whether the value is updated or not. If the XRT computation is
  + // going to be used with a TensorFlow TPU execution op then an
  + // input index must be present for each input that will correspond
  + // to a resource variable in the execution op, and may not be
  + // present for any other input.
  + repeated UpdateIndexPair stateful_input_indices = 1;
  + }
  +
  + Experimental experimental = 8;
  }
  // Options and XLA computation for a compilation.
  