Simplify check_requirements() usage (#4855)

* Simplify `check_requirements()` usage

* remove assert, print()

train.py

@@ -476,7 +476,7 @@ def main(opt, callbacks=Callbacks()):
     if RANK in [-1, 0]:
         print_args(FILE.stem, opt)
         check_git_status()
-        check_requirements(requirements=ROOT / 'requirements.txt', exclude=['thop'])
+        check_requirements(exclude=['thop'])
 
     # Resume
     if opt.resume and not check_wandb_resume(opt) and not opt.evolve:  # resume an interrupted run