DenseCL weight initialization inconsistent with original code.

[lorinczszabolcs]

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Hi!

I noticed, that the weight initialization in DenseCL is inconsistent with the original code, namely that when pre-trained weights are loaded, only the query encoder will get initialized, but the weights won't be copied to the key encoder, while in the original repo it is done like that, see https://github.com/WXinlong/DenseCL/blob/360df04ba25aca36d42e85c5a96552783568fccf/openselfsup/models/densecl.py#L36 and https://github.com/WXinlong/DenseCL/blob/360df04ba25aca36d42e85c5a96552783568fccf/openselfsup/models/densecl.py#L51-L58.

I think DenseCL's init_weights should be overrided by having

def init_weights(self):
    """Init weights and copy query encoder init weights to key encoder."""
    super().init_weights()
    for param_q, param_k in zip(self.encoder_q.parameters(),
                                self.encoder_k.parameters()):
        param_k.data.copy_(param_q.data)
        param_k.requires_grad = False