handle out is none

Author: Ali Alshaarawy

thunder/core/rematerialization.py

@@ -695,7 +695,7 @@ def add_to_swapmap(p):
         if bsym.sym.id == PrimIDs.UNPACK_SEQUENCE:
             flattened_args = tree_flatten(bw_trace.args[1])[0]
             proxy_names = {y.name for y in flattened_args if isinstance(y, ProxyInterface)}
-            if all(not isinstance(out, CollectionProxy) and out.name not in proxy_names for out in bsym.flat_outs):
+            if all(not isinstance(out, CollectionProxy) and out.name not in proxy_names for out in bsym.flat_outs if out is not None):
                 old_saved_for_backward_bw += bsym.flat_outs
     assert len(old_saved_for_backward_fw) == len(old_saved_for_backward_bw)
     new_required_for_bakward_fw_to_bw_map = {