Dim get_size_dim_tag fix (#1148)

Also fixes Dim.is_dynamic. Related: #1139

Also needed for #1143.

Author: albertz

returnn/tf/layers/rec.py

@@ -1886,9 +1886,7 @@ def get_layer(name):
         prev_layer = prev_layers[layer_name]
         assert layer.output.batch_shape == prev_layer.output.batch_shape
         assert layer.output.batch_dim_axis == prev_layer.output.batch_dim_axis
-        assert sorted(layer.output.size_placeholder.keys()) == sorted(prev_layer.output.size_placeholder.keys())
-        for i in range(len(layer.output.size_placeholder)):
-          assert layer.output.get_size_dim_tag(i) == prev_layer.output.get_size_dim_tag(i)
+        assert layer.output.get_dyn_size_tags() == prev_layer.output.get_dyn_size_tags()
 
   def get_prev_template_layer(self, layer_name):
     """

returnn/tf/util/data.py

@@ -518,7 +518,7 @@ def is_dynamic(self):
     :return: whether the dim is not static. usually means that it has seq lengths
     :rtype: bool
     """
-    return self.dimension is not None
+    return self.dimension is None and not self.is_batch_dim()
 
   def can_be_used_as_dim(self):
     """
@@ -5412,13 +5412,20 @@ def get_time_dim_tag(self):
     assert self.time_dim_axis is not None
     return self.get_dim_tag(self.time_dim_axis)
 
+  def get_dyn_size_tags(self):
+    """
+    :return: all dim tags with dynamic size
+    :rtype: list[Dim]
+    """
+    return [dim_tag for dim_tag in self._dim_tags if dim_tag.is_dynamic()]
+
   def get_size_dim_tag(self, number):
     """
     :param int number: index in sorted(size_placeholder.keys())
     :rtype: Dim
     """
-    axis_wo_batch = sorted(self.size_placeholder.keys())[number]
-    return self.get_dim_tag(self.get_batch_axis(axis_wo_batch))
+    dyn_size_tags = self.get_dyn_size_tags()
+    return dyn_size_tags[number]
 
   def get_batch_shape_dim_tags(self):
     """