change file name to snake style (#47914)
Summary:
Change Partitioner.py file name to partitioner.py
Change GraphManipulation.py file name to graph_manipulation.py
Move test_replace_target_nodes_with() to test_fx_experimental.py
Remove the unnecessary argument in size_based_partition() in Partitioner class
Pull Request resolved: https://github.com/pytorch/pytorch/pull/47914
Reviewed By: gcatron
Differential Revision: D24956653
Pulled By: scottxu0730
fbshipit-source-id: 25b65be7dc7d64e90ffdc59cf394446fee83c3e6
