Fix assert on tuple (#692)
Summary:
Because of the parentheses assert was running on tuple, and a non-empty tuple is always True, so the assert would not catch empty `json_files`.
An example:
>>> assert(False, f"Assertion error!") # Assertion error will not be thrown!
<stdin>:1: SyntaxWarning: assertion is always true, perhaps remove parentheses?
Pull Request resolved: https://github.com/pytorch/benchmark/pull/692
Reviewed By: xuzhao9
Differential Revision: D33552198
Pulled By: kit1980
fbshipit-source-id: 0a9d0ca3b20ef8595f9efffaa6df65ddf03d2a6b
