Fix lint error in format_time() in throughput_benchmark.py and clean it up a bit. (#22424)
Summary:
The `assert False` lint error has been causing CI to fail:
./torch/utils/throughput_benchmark.py:14:13: B011 Do not call assert False since python -O removes these calls. Instead callers should raise AssertionError().
Pull Request resolved: https://github.com/pytorch/pytorch/pull/22424
Differential Revision: D16083464
Pulled By: bddppq
fbshipit-source-id: 6d96e36c8fcbb391d071b75fe79c22d526c1ba3c
