Update train.py for val.run(half=amp) (#8804)

Disable FP16 validation if AMP checks fail or amp=False.

train.py

@@ -367,6 +367,7 @@ def train(hyp, opt, device, callbacks):  # hyp is path/to/hyp.yaml or hyp dictio
                 results, maps, _ = val.run(data_dict,
                                            batch_size=batch_size // WORLD_SIZE * 2,
                                            imgsz=imgsz,
+                                           half=amp,
                                            model=ema.ema,
                                            single_cls=single_cls,
                                            dataloader=val_loader,