[Transform] add infer_shapes

currently wrapper for ONNX shape inference, will change later

src/finn/transformation/infer_shapes.py

+import onnx.shape_inference as si
+
+
+def infer_shapes(model):
+    """Ensure every tensor in the model has a specified shape (ValueInfo)."""
+    # currently just calls ONNX shape inference, but in the future we will
+    # have to handle shape inference for custom ops ourselves
+    model.model = si.infer_shapes(model.model)
+    # single-step operation, no need to call multiple times so return
+    # model_was_changed = false
+    return (model, False)