I have solved this problem, which is actually caused by  this line of code in the _load_from_state_dict function:
                if input_param.shape != param.shape:
                    # local shape should match the one in checkpoint
                    error_msgs.append('size mismatch for {}: copying a param with shape {} from checkpoint, '
                                      'the shape in current model is {}.'
                                      .format(key, input_param.shape, param.shape))

It just means the param shape in your checkpoint file([1,64]) is deferent from that in your model you define([64]). So what shoule you do is just modify the  param of conv1_7x7_s2_bn layers or modify your checkpoint file. This is just my personal suggestioin , hope it help.