call after_val_epoch hooks only once to save best ckpt correctly

[Weiyun1025]

Motivation

SingleTeacherDistillValLoop calls after_val_epoch hooks twice during the evaluation. However, the metric in the second call do not contain the key named student.accuracy/top1. As a result, if we set save_best='auto' in CheckpointHook, the program will fail.

Please refer to this issue for more details.

Modification

Now we only call after_val_epoch hooks once in SingleTeacherDistillValLoop.

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