move median to ATen (#17637)
Summary:
This moves median to ATen.
- median with dimension reduces to kthvalue
- median without dimension (aka medianall) is implemented in parallel to kthvalue because we would not want to reshape (copying for non-contiguous) and then copy again in kthvalue. We can sue the helper functions we moved from kthvalue.
- `median_cuda` was accidentally already put into ATen in #17544.
- The quickselect algorthm without indices for CPU in TH is now obsolete and removed.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/17637
Differential Revision: D14346510
Pulled By: ezyang
fbshipit-source-id: c07ad144efbd6b4194179bb1c02635862521d8cb
