Adding support for manifold files in DBReader (#37727)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/37727
Check if the file exists locally only for `log_file_db` db_type. Reader files in other `db_type` like `manifold_log_file_db` are excluded from this check.
Test Plan: Verified that files stored in manifold can be loaded using `DBFileReader`.
Reviewed By: hbjerry
Differential Revision: D21329671
fbshipit-source-id: bbc0e88851783ca3f78f7c61bfe84b480c09b5ac
