add benchmark utility for datasets (#4555)
add benchmark utility for datasets (#4555)

* add benchmark utility for datasets

* cleanup

* cleanup

* add benchmarks for MNIST and variants

* simplifiy return if exeception

* refactor benchmark instantiation logic

Co-authored-by: Prabhat Roy <de68f51b8671e71a0285845858f23ed328c37e06@fb.com>
pytorch/vision,pytorch/vision,pytorch/vision,pytorch/vision,pytorch/vision,pytorch/vision