Add _native nested_tensor_from_mask (#76942)
Summary: For user to convert nested tensor more easily. Some impl detail might change on user's need.
Test Plan: buck test mode/dev caffe2/test:nn -- test_nested_tensor_from_mask
Differential Revision: D36191182
Pull Request resolved: https://github.com/pytorch/pytorch/pull/76942
Approved by: https://github.com/jbschlosser
