Pruning_test.py error: module 'pruning' has no attribute 'union' and 'intersect'

[Jiacheng98]

Two methods:

1. Add "union" and "intersect" functions (in union.py) to pruning.py
   or
2. Add "from lottery_ticket.foundations import union" line to pruning_test.py
3. Change line 33 "union_mask = pruning.union(masks1, masks2)" to "union_mask = union.union(masks1, masks2)",
   and line 39 "intersect_mask = pruning.intersect(masks1, masks2)" to "intersect_mask = union.intersect(masks1, masks2)"