Prevent tf1 loop ops from const folding (#1561)

Signed-off-by: Tom Wildenhain <[email protected]>

Author: TomWildenhain-Microsoft

tf2onnx/tf_utils.py

@@ -216,7 +216,8 @@ def is_huge_shape(x):
                     outputs_to_values[output_names[0]] = np.array(shape[i], dtype=np_dtype)
                     outputs_to_dtypes[node.outputs[0].name] = node.outputs[0].dtype
                     progress = True
-            can_fold = node.type not in ['Enter', 'Placeholder', 'PlaceholderWithDefault']
+            can_fold = node.type not in ['Enter', 'Placeholder', 'PlaceholderWithDefault', 'Switch', 'Merge',
+                                         'NextIteration', 'Exit']
             can_fold = can_fold and not node.type.startswith('Random')
             can_fold = can_fold and len(input_names) > 0 and all(inp in outputs_to_values for inp in input_names)
             # We can only fold nodes with a single output