add list methods: copy,extend (#17092)
Summary:
This PR adds the following methods to python's list.
* copy
* extend
and tests
Pull Request resolved: https://github.com/pytorch/pytorch/pull/17092
Differential Revision: D14141817
Pulled By: Krovatkin
fbshipit-source-id: c89207f0f25f3d1d4ad903ee634745615d61d576
