Set default topk when num_classes < 5 (#269)

* set default topk when num_classes < 6

* add comments

* format changes with yapf

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Co-authored-by: Xu Wenqing <xwq391974@alibaba-inc.com>

modelscope/trainers/cv/image_classifition_trainer.py

@@ -498,6 +498,11 @@ class ImageClassifitionTrainer(BaseTrainer):
             metric_options = self.cfg.evaluation.get('metric_options', {})
             if 'topk' in metric_options.keys():
                 metric_options['topk'] = tuple(metric_options['topk'])
+            # mmcls will set the default value of topk to (1, 5) which
+            # will cause error when number of classes less then 5.
+            # set topk as (1,) if len(CLASSES) < 5:
+            elif len(CLASSES) < 5:
+                metric_options['topk'] = (1, )
             if self.cfg.evaluation.metrics:
                 eval_results = dataset.evaluate(
                     results=outputs,