add command line option to use hive filler; add README (#17619)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/17619
--filler hive --iter -1 will let debugger exhaust all batches from a hive partition before exiting.
add README that summarizes command line options and usage.
Reviewed By: yinghai
Differential Revision: D14220166
fbshipit-source-id: daa23b7e8a9184481c6d7b67acf1599e5c99d74a
