Fix prev_iter (pytorch#868)

Summary:
Pull Request resolved: facebookresearch/FBGEMM#868

X-link: pytorch#3784

as titled.

Reviewed By: q10, sryap

Differential Revision: D70794993

fbshipit-source-id: 07004d78a6112c972d77a3fdf0c74519dcd4019b

Author: spcyppt

fbgemm_gpu/codegen/training/python/split_embedding_codegen_lookup_invoker.template

@@ -213,7 +213,7 @@ def invoke(
     {%- if "prev_iter" not in args_pt2.unified_pt2.split_unpacked_arg_names %}
     dict_aux_tensor["prev_iter_dev"] = prev_iter_dev
     {%- else %}
-    dict_aux_tensor["prev_iter_dev"] = prev_iter.prev_iter_dev
+    dict_aux_tensor["prev_iter_dev"] = prev_iter.dev
     {%- endif %}
 
     # optimizer_args