val.py `--weights` and `--data` compatibility check (#7292)

Improved error messages for understanding of user error with val.py. May help https://github.com/ultralytics/yolov5/issues/7291

val.py

@@ -162,6 +162,10 @@ def run(
 
     # Dataloader
     if not training:
+        if pt and not single_cls:  # check --weights are trained on --data
+            ncm = model.model.yaml['nc']
+            assert ncm == nc, f'{weights[0]} ({ncm} classes) trained on different --data than what you passed ({nc} ' \
+                              f'classes). Pass correct combination of --weights and --data that are trained together.'
         model.warmup(imgsz=(1 if pt else batch_size, 3, imgsz, imgsz))  # warmup
         pad = 0.0 if task in ('speed', 'benchmark') else 0.5
         rect = False if task == 'benchmark' else pt  # square inference for benchmarks