Use defaultdict to avoid hard coding buffer keys in Concatenator and ConcatDataset

[bofenghuang]

What does this PR do?

Use defaultdict to avoid hard coding buffer keys in Concatenator and ConcatDataset.

Before submitting

• This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
• Did you read the contributor guideline,
  Pull Request section?
• Was this discussed/approved via a Github issue? Please add a link
  to it if that's the case.
• Did you make sure to update the documentation with your changes?
• Did you write any new necessary tests?

Thanks for contributing 🎉!