Allow placeholders in Graph's init scope (#393)

Signed-off-by: Ryan Nett <[email protected]>

Author: rnett

tensorflow-core/tensorflow-core-api/src/main/java/org/tensorflow/Graph.java

@@ -588,10 +588,6 @@ private boolean registerInitOpHelper(Operation op) {
     }
     GraphOperation graphOp = (GraphOperation) op;
 
-    if (op.type().equals(Placeholder.OP_NAME)) {
-      throw new IllegalStateException("Can not make a placeholder " + op + " an init op.");
-    }
-
     for (GraphOperation controlInput : graphOp.controlInputs()) {
       registerInitOpHelper(controlInput);
     }