use deterministic dim order in stack

Author: Joseph Hamman

xbatcher/generators.py

@@ -55,7 +55,7 @@ def _drop_input_dims(ds, input_dims, suffix='_input'):
 
 
 def _maybe_stack_batch_dims(ds, input_dims, stacked_dim_name='sample'):
-    batch_dims = list(set(ds.dims) - set(input_dims))
+    batch_dims = [d for d in ds.dims if d not in input_dims]
     if len(batch_dims) < 2:
         return ds
     ds_stack = ds.stack(**{stacked_dim_name: batch_dims})