Replace get_arguments().at(x) with get_argument(x)

This came up while debugging memory corruption in the ngraph-tf tests: plaidml_ops_replicate was holding onto
a reference to a temporary that was destroyed while the reference was still live.  Replacing that code with a
call to get_argument() eliminated the temporary and fixed the issue.  And in general, there's no reason to be
allocating temporary vectors when we're pulling out a single argument, so this change replaces the other
sequences of get_arguments().at(), too.

src/ngraph/runtime/plaidml/plaidml_ops_replicate.cpp

@@ -53,7 +53,7 @@ ngraph::runtime::plaidml::op::Replicate::Replicate(std::shared_ptr<Node> arg,
 
 void ngraph::runtime::plaidml::op::Replicate::validate_and_infer_types()
 {
-    const auto& arg = get_arguments().at(0);
+    std::shared_ptr<Node> arg = get_argument(0);
     Shape shape = arg->get_shape();
     for (auto rit = m_replication_axes.begin(), sit = shape.begin();
          rit != m_replication_axes.end();

src/ngraph/runtime/plaidml/plaidml_pass_lower_convolutions.cpp

@@ -75,19 +75,19 @@ ngraph::runtime::plaidml::pass::LowerConvolutions::LowerConvolutions()
         // op.  Using target always works.
         AxisVector out_axes = to_axes(target, output_transpose);
 
-        auto lhs = node->get_arguments().at(0);
+        auto lhs = node->get_argument(0);
         auto* lhs_transpose = to_transpose(lhs);
         if (lhs_transpose)
         {
-            lhs = lhs_transpose->get_arguments().at(0);
+            lhs = lhs_transpose->get_argument(0);
         }
         AxisVector lhs_axes = to_axes(lhs, lhs_transpose);
 
-        auto rhs = node->get_arguments().at(1);
+        auto rhs = node->get_argument(1);
         auto* rhs_transpose = to_transpose(rhs);
         if (rhs_transpose)
         {
-            rhs = rhs_transpose->get_arguments().at(0);
+            rhs = rhs_transpose->get_argument(0);
         }
         AxisVector rhs_axes = to_axes(rhs, rhs_transpose);
 

src/ngraph/runtime/plaidml/plaidml_pass_replicate_combination.cpp

@@ -48,7 +48,7 @@ ngraph::runtime::plaidml::pass::ReplicateCombination::ReplicateCombination()
         }
 
         replace_node(lower,
-                     std::make_shared<plaidml::op::Replicate>(upper->get_arguments().at(0),
+                     std::make_shared<plaidml::op::Replicate>(upper->get_argument(0),
                                                               std::move(axes)));
 
         return true;

src/ngraph/runtime/plaidml/plaidml_pass_replicate_elision.cpp

@@ -74,7 +74,7 @@ ngraph::runtime::plaidml::pass::ReplicateElision::ReplicateElision()
             if (elidable)
             {
                 replaced_any = true;
-                replace_node(replicate, replicate->get_arguments().at(0));
+                replace_node(replicate, replicate->get_argument(0));
             }
         }