Fix issue with importing models using Tensorflow Lite 2.4.x schema (a…

…pache#8375)

Tensorflow Lite has changed the opcode for BuiltinOperators
to be represented as 32 bit integers instead of 8 bit integers
in the schema.

This is an attempt to fix this in a way that is clean to handle
multiple versions of tensorflow lite in the frontend.

python/tvm/relay/frontend/tflite.py

@@ -251,7 +251,30 @@ def get_op_code_str(self, op):
             raise ImportError("The tflite package must be installed")
 
         op_code_list_idx = op.OpcodeIndex()
-        op_code_id = self.model.OperatorCodes(op_code_list_idx).BuiltinCode()
+
+        op_c = self.model.OperatorCodes(op_code_list_idx)
+        # In TFlite 2.4.x there was a change where the type of the field that contained
+        # the builtin code changed from int8 to int32 in the flat buffer representation.
+        # However to retain support for old flat buffers that were created, they retained
+        # the original 8 bit encoding for the operator but in a new field accessed by the
+        # DeprecatedBuiltinCode method.
+        # This means that the API function BuiltinCode() is used on an operator
+        # which was originally encoded as an 8 bit quantity it would look for the
+        # code in the new int32 field in the schema and this creates the need
+        # for the check for the magic number of 127 which is indicated by
+        # BuiltinOperator.PLACEHOLDER_FOR_GREATER_OP_CODES
+        # Remember however that this value came into existence only after Tensorflow
+        # lite 2.4.x and hence encase it in a try -except block.
+        # Phew !
+        try:
+            if op_c.BuiltinCode() < BuiltinOperator.PLACEHOLDER_FOR_GREATER_OP_CODES:
+                opc = op_c.DeprecatedBuiltinCode()
+            else:
+                opc = op_c.BuiltinCode()
+        except AttributeError:
+            opc = op_c.BuiltinCode()
+
+        op_code_id = opc
         try:
             op_code_str = self.builtin_op_code[op_code_id]
         except KeyError: