message: Re-enable the marking of backward matmul operations
  Summary:
  Marking of passes as backward was disabled due to a bug in poplibs. That
  bug is fixed, so this change re-enables the marking.
  Test Plan: All existing tests passing
  Reviewers: georgep
diff: @@ -151,20 +151,17 @@ bool IsForwardMatMul(const HloInstruction *inst) {
  }
  bool IsGradientMatMul(const HloInstruction *inst) {
  - // const HloInstruction* lh = inst->operand(0);
  - // const HloInstruction* rh = inst->operand(1);
  - // return (lh->opcode() != HloOpcode::kTranspose &&
  - // rh->opcode() == HloOpcode::kTranspose);
  - return false;
  + const HloInstruction* lh = inst->operand(0);
  + const HloInstruction* rh = inst->operand(1);
  + return (lh->opcode() != HloOpcode::kTranspose &&
  + rh->opcode() == HloOpcode::kTranspose);
  }
  bool IsWeightUpdateMatMul(const HloInstruction *inst) {
  - // const HloInstruction* lh = inst->operand(0);
  - // const HloInstruction* rh = inst->operand(1);
  - // return (lh->opcode() == HloOpcode::kTranspose &&
  - // rh->opcode() != HloOpcode::kTranspose);
  - // Cannot mark as MATMUL WU until poplibs T2934 fixed
  - return false;
  + const HloInstruction* lh = inst->operand(0);
  + const HloInstruction* rh = inst->operand(1);
  + return (lh->opcode() == HloOpcode::kTranspose &&
  + rh->opcode() != HloOpcode::kTranspose);
  }
  bool IsTfReluGradOp(const HloInstruction *inst) {
  