Fix 6 Flake8 issues (#6541)

* F541

* F821

* F841

* E741

* E302

* E722

* Apply suggestions from code review

* Update general.py

* Update datasets.py

* Update export.py

* Update plots.py

* Update plots.py

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

models/tf.py

@@ -427,13 +427,13 @@ def run(weights=ROOT / 'yolov5s.pt',  # weights path
     # PyTorch model
     im = torch.zeros((batch_size, 3, *imgsz))  # BCHW image
     model = attempt_load(weights, map_location=torch.device('cpu'), inplace=True, fuse=False)
-    y = model(im)  # inference
+    _ = model(im)  # inference
     model.info()
 
     # TensorFlow model
     im = tf.zeros((batch_size, *imgsz, 3))  # BHWC image
     tf_model = TFModel(cfg=model.yaml, model=model, nc=model.nc, imgsz=imgsz)
-    y = tf_model.predict(im)  # inference
+    _ = tf_model.predict(im)  # inference
 
     # Keras model
     im = keras.Input(shape=(*imgsz, 3), batch_size=None if dynamic else batch_size)