Fix API tflite.OperatorCode.BuiltinCode() compatibility issue

`tflite.OperatorCode.BuiltinCode()`: maintains API compability in 2.4.0.
See these threads:
* #9
* #10
* tensorflow/tensorflow#46663

tests/test_mobilenet.py

@@ -64,13 +64,10 @@ def test_mobilenet():
     op_code = model.OperatorCodes(op.OpcodeIndex())
 
     # The first operator is a convolution.
-    # The builtin code is extended in TensorFlow 2.4.x from 8 bit to 32 bit.
-    # See this example of how to handle it gracefully in your code
-    # https://github.com/apache/tvm/blob/b20b7c4ad4ad3774a42f47614245f8eeabe875cb/python/tvm/relay/frontend/tflite.py#L297-L316
-    assert(op_code.DeprecatedBuiltinCode() == tflite.BuiltinOperator.CONV_2D)
+    assert(op_code.BuiltinCode() == tflite.BuiltinOperator.CONV_2D)
 
     # Custom operator need more interface, won't cover here.
-    assert(op_code.DeprecatedBuiltinCode() != tflite.BuiltinOperator.CUSTOM)
+    assert(op_code.BuiltinCode() != tflite.BuiltinOperator.CUSTOM)
 
 
     ############# the operator ##################################################

tflite/OperatorCode.py

@@ -53,8 +53,13 @@ def Version(self):
     def BuiltinCode(self):
         o = flatbuffers.number_types.UOffsetTFlags.py_type(self._tab.Offset(10))
         if o != 0:
-            return self._tab.Get(flatbuffers.number_types.Int32Flags, o + self._tab.Pos)
-        return 0
+            o = self._tab.Get(flatbuffers.number_types.Int32Flags, o + self._tab.Pos)
+
+        from tflite.BuiltinOperator import BuiltinOperator
+        if o < BuiltinOperator.PLACEHOLDER_FOR_GREATER_OP_CODES:
+            return self.DeprecatedBuiltinCode()
+        else:
+            return o
 
 def OperatorCodeStart(builder): builder.StartObject(4)
 def Start(builder):
@@ -73,4 +78,4 @@ def AddBuiltinCode(builder, builtinCode):
     return OperatorCodeAddBuiltinCode(builder, builtinCode)
 def OperatorCodeEnd(builder): return builder.EndObject()
 def End(builder):
-    return OperatorCodeEnd(builder)
+    return OperatorCodeEnd(builder)