You would have to access the internal attribute through the .datasets attribute as seen here:
# setup
datasetA = TensorDataset(torch.randn(10, 1), torch.randn(10, 1))
datasetB = TensorDataset(torch.randn(10, 1), torch.randn(10, 1))

# access internal attribute
datasetA.tensors[0].shape

# concat
dataset = ConcatDataset((datasetA, datasetB))

# access attribute through the internal datasets
dataset.datasets[0].tensors[0].shape