Write add_scalars data in the same file (#21100)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/21100
Added multifile flag to write scalar data into separate files. This can slow down dashboard loading.
Reviewed By: orionr
Differential Revision: D15548913
fbshipit-source-id: dd39a7f76f93025d28f14babbf933e39860e6910
