Enhance `DatasetSummary` for several points

[Nic-Ma]

Is your feature request related to a problem? Please describe.
Thanks for @dongyang0122 's great work to develop DatasetSummary, would be nice to enhance the below items:

1. It only has meta_key_postfix arg, missed meta_key to be consistent with other MONAI modules when using flexible meta key: https://github.com/Project-MONAI/MONAI/blob/dev/monai/transforms/io/dictionary.py#L68
2. The spacing data or image, label may not be Tensor, it depends on the Dataset and collate_fn of DataLoader, need to convert data type to Tensor first.
3. It supports kwargs for DataLoader but didn't cover it in the unit tests, would be nice to add it.