Hotfix: Circumvent tf-2.12 breaking change on tflite subgraph API to unbreak UT

TF-2.12.0 introduced API change that breaks tf2onnx UT tests on the
tflite paths, due to the addition of compulsory subgraph arg to several
function's input signature:
tensorflow/tensorflow@55d84d7

This commit is a temporary hotfix to unbreak related UT failure.
Existing tf2onnx's use cases get tflite Interpreter's tensors from model's
first subgraph only. The hotfix hard-codes subgraph index to `0` to
retain the same behavior while resolves API diff.

Author: Yu Cong

tests/backend_test_base.py

@@ -252,11 +252,12 @@ def tflite_has_supported_types(self, tflite_path):
             with open(tflite_path, 'rb') as f:
                 buf = f.read()
             buf = bytearray(buf)
+            subgraph_idx = 0 # assume only one subgraph
             model = Model.GetRootAsModel(buf, 0)
-            tensor_cnt = model.Subgraphs(0).TensorsLength()
+            tensor_cnt = model.Subgraphs(subgraph_idx).TensorsLength()
             interpreter = tf.lite.Interpreter(tflite_path)
             for i in range(tensor_cnt):
-                dtype = interpreter._get_tensor_details(i)['dtype']   # pylint: disable=protected-access
+                dtype = interpreter._get_tensor_details(i, subgraph_idx)['dtype']   # pylint: disable=protected-access
                 if np.dtype(dtype).kind == 'O':
                     return False
             return True

tf2onnx/tflite_utils.py

@@ -196,10 +196,11 @@ def read_tflite_model(tflite_path):
     try:
         interpreter = tf.lite.Interpreter(tflite_path)
         interpreter.allocate_tensors()
-        tensor_cnt = model.Subgraphs(0).TensorsLength()
+        subgraph_idx = 0 # assume only one subgraph
+        tensor_cnt = model.Subgraphs(subgraph_idx).TensorsLength()
         for i in range(tensor_cnt):
-            name = model.Subgraphs(0).Tensors(i).Name().decode()
-            details = interpreter._get_tensor_details(i)   # pylint: disable=protected-access
+            name = model.Subgraphs(subgraph_idx).Tensors(i).Name().decode()
+            details = interpreter._get_tensor_details(i, subgraph_idx)   # pylint: disable=protected-access
             if "shape_signature" in details:
                 tensor_shapes[name] = details["shape_signature"].tolist()
             elif "shape" in details: