Fix and add testing for nullptr allocator in c2->pt conversion (#16857)
Summary:
Fixes the bug for when tensor is created on Caffe2 side, then passed to PT and resized. Now we just initialize allocator correctly.
Note that the code in raw_mutable_data() is still necessary because of non-resizable tensors.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/16857
Reviewed By: houseroad
Differential Revision: D14019469
Pulled By: dzhulgakov
fbshipit-source-id: 14d3a3b946d718bbab747ea376903646b885706a
