Reducing excessive control dependencies from inplacing

Summary:
Preventing inplace finder from creating unneeded control dependencies which sometimes resulted in cycles.

  - Operation `A`, has two inplace read/write users `B` and `C`.
  - Inplace finder looks at inplace op `B` first, and adds control dependencies to guarantee it happens after all of its peers (other users of its inplace operand, in this case `C`).
  - Next inplace finder looks at inplace op `C`. It sees that one of its peers (`B`) is reachable from it (because of the control dependency) so the situation cannot be resolved by inserting control dependencies. Instead it inserts a copy (`A` -> `copy` -> `C`).
  - Previously `copy` copied control successors from `A`, and control predecessors from `C`. Both of these are unnecessary.

Test Plan: Existing inplace tests.

Reviewers: #tensorflow, #framework_ip_review_-_any_oss_or_third-party_code_use_has_been_approved, samuelh

Reviewed By: #tensorflow, #framework_ip_review_-_any_oss_or_third-party_code_use_has_been_approved, samuelh

Maniphest Tasks: T66881

Differential Revision: https://phabricator.sourcevertex.net/D74373

Author: Alfie-Edwards

tensorflow/compiler/plugin/poplar/driver/passes/inplace_finder.cc

@@ -377,17 +377,6 @@ StatusOr<bool> InsertCopies(InplacingState& state) {
                       copy->shape(), CloneMethod_PreserveOrderUnlessAliases)));
       }
 
-      // If we copy result of the instruction, we have to guarantee that
-      // result was copied before any control successors (that potentially may
-      // modify this buffer inplace).
-      for (auto succ : op->control_successors()) {
-        TF_RETURN_IF_ERROR(copy->AddControlDependencyTo(succ));
-      }
-      // If the instruction has control predecessors, we have to guarantee
-      // that copies are made after all predecessors.
-      for (HloInstruction* pred : inst->control_predecessors()) {
-        TF_RETURN_IF_ERROR(pred->AddControlDependencyTo(copy));
-      }
       op->SetupDerivedInstruction(copy);
       TF_RETURN_IF_ERROR(inst->ReplaceOperandWith(op_index, copy));
       changed = true;