Model `ema` key backward compatibility fix (#6972)

Fix for older model loading issue in https://github.com/ultralytics/yolov5/commit/d3d9cbce221b2ced46dde374f24fde72c8e71c37#commitcomment-68622388

models/experimental.py

@@ -94,7 +94,7 @@ def attempt_load(weights, map_location=None, inplace=True, fuse=True):
     model = Ensemble()
     for w in weights if isinstance(weights, list) else [weights]:
         ckpt = torch.load(attempt_download(w), map_location=map_location)  # load
-        ckpt = (ckpt['ema'] or ckpt['model']).float()  # FP32 model
+        ckpt = (ckpt.get('ema') or ckpt['model']).float()  # FP32 model
         model.append(ckpt.fuse().eval() if fuse else ckpt.eval())  # fused or un-fused model in eval mode
 
     # Compatibility updates