Revert batched pdist, improve existing kernel, add test (#15901)

Revert batched pdist, improve existing kernel, add test (#15901)

Summary:
1) Reverts https://github.com/pytorch/pytorch/pull/12302 which added support for batched pdist. Except I kept the (non-batched) test improvements that came with that PR, because they are nice to have.  Motivation: https://github.com/pytorch/pytorch/issues/15511
2) For the non-batched pdist, improved the existing kernel by forcing fp64 math and properly checking cuda launch errors
3) Added a 'large tensor' test that at least on my machine, fails on the batch pdist implementation.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/15901

Reviewed By: ezyang

Differential Revision: D13616730

Pulled By: gchanan

fbshipit-source-id: 620d3f9b9acd492dc131bad9d2ff618d69fc2954