test RF utils, allow without TF

tests/rf_utils.py

@@ -8,19 +8,27 @@
 import re
 import numpy
 import numpy.testing
-import tensorflow as tf
+
+try:
+    import tensorflow as tf
+except ImportError:
+    tf = globals().get("tf", None)  # type: ignore
 
 from returnn.config import Config, global_config_ctx
 from returnn.util.pprint import pprint
 import returnn.frontend as rf
 from returnn.tensor import Tensor, Dim, TensorDict
 from returnn.tensor.utils import tensor_dict_fill_random_numpy_
-import returnn.tf.compat as tf_compat
 import returnn.torch.frontend as rft
-import returnn.tf.frontend_layers as rfl
-from returnn.tf.network import TFNetwork
 from returnn.torch.data.tensor_utils import tensor_dict_numpy_to_torch_, tensor_dict_torch_to_numpy_
 
+if tf:
+    import returnn.tf.compat as tf_compat
+    import returnn.tf.frontend_layers as rfl
+    from returnn.tf.network import TFNetwork
+else:
+    tf_compat = rfl = TFNetwork = None
+
 # noinspection PyProtectedMember
 from returnn.frontend._random_journal import RandomJournal