message: [XLA] Fix comments on HloInstruction::epsilon() and HloInstruction::feature_index().
  These functions can be called for kBatchNorm{Training,Inference,Grad},
  not just kBatchNormTraining.
  No functional change.
diff: @@ -765,13 +765,15 @@ class HloInstruction {
  // Returns feature_index field associated with the instruction. The index
  // represents the index of the feature dimension.
  //
  - // Precondition: opcode() == HloOpcode::kBatchNormTraining
  + // Precondition: opcode() is one of kBatchNormTraining, kBatchNormInference,
  + // or kBatchNormGrad.
  int64 feature_index() const { return feature_index_; }
  // Returns a epsilon value associated with the instruction. The is a small
  // number added to the variance to avoid divide-by-zero error.
  //
  - // Precondition: opcode() == HloOpcode::kBatchNormTraining
  + // Precondition: opcode() is one of kBatchNormTraining, kBatchNormInference,
  + // or kBatchNormGrad.
  float epsilon() const { return epsilon_; }
  // Returns the infeed configuration string. The infeed configuration includes
  