当我为以下变量打印张量形状时，
self.tensor_image = self.graph.get_tensor_by_name('TfPoseEstimator/image:0')
self.tensor_output = self.graph.get_tensor_by_name('TfPoseEstimator/Openpose/concat_stage7:0')
self.tensor_heatMat = self.tensor_output[:, :, :, :19]
self.tensor_pafMat = self.tensor_output[:, :, :, 19:]
self.tensor_output has (?, ?, ?, 57)
self.tensor_heatMat has (?, ?, ?, 19)
self.tensor_pafMat has (?, ?, ?, 38)
这些?是什么意思？