[NNVM] TF, Prefer Placeholders to Consts when processing inputs

This patch disable setting first Const as input if there are some
Placeholders in Graph. It allows to parse some real-world models
generated by e.g. `tf.simple_save` in cases when Placeholders were
inserted _after_ Constants.

nnvm/python/nnvm/frontend/tensorflow.py

@@ -888,10 +888,11 @@ def _parse_import_prerequisites(graph):
      b. Set of operator names which don't have their mapping in TVM, i.e.
         which are not supported
     """
+    placeholders = []
     missing_operators = set()
     for node in graph.node:
         if node.op == "Placeholder":
-            pass
+            placeholders.append(node)
         elif node.op == "Const":
             pass
         else:
@@ -900,7 +901,7 @@ def _parse_import_prerequisites(graph):
             else:
                 missing_operators.add(node.op)
 
-    return missing_operators
+    return (placeholders,missing_operators)
 
 
 class GraphProto(object):
@@ -955,7 +956,7 @@ def from_tensorflow(self, graph):
             raise ImportError(
                 "Unable to import tensorflow which is required {}".format(e))
 
-        missing_operators = _parse_import_prerequisites(graph)
+        placeholders,missing_operators = _parse_import_prerequisites(graph)
 
         if missing_operators:
             raise NotImplementedError( \
@@ -981,7 +982,7 @@ def from_tensorflow(self, graph):
                     raise NotImplementedError( \
                         "Please freeze the graph with add_shapes=True")
             elif node.op == "Const":
-                if self._input_node == '':
+                if self._input_node == '' and not placeholders:
                     self._input_node = node.name
                     self._num_input += 1
                     self._nodes[node.name] = _sym.Variable(name=node.name)