Explicit `model.eval()` call `if opt.train=False` (#3475)

* call model.eval() when opt.train is False

call model.eval() when opt.train is False

* single-line if statement

* cleanup

Co-authored-by: Glenn Jocher <glenn.jocher@ultralytics.com>

models/export.py

@@ -58,8 +58,7 @@ if __name__ == '__main__':
     # Update model
     if opt.half:
         img, model = img.half(), model.half()  # to FP16
-    if opt.train:
-        model.train()  # training mode (no grid construction in Detect layer)
+    model.train() if opt.train else model.eval()  # training mode = no Detect() layer grid construction
     for k, m in model.named_modules():
         m._non_persistent_buffers_set = set()  # pytorch 1.6.0 compatibility
         if isinstance(m, models.common.Conv):  # assign export-friendly activations