Make is_sparse a property of MaskedTensor (#110725)
Fixes #104574
Seeing that MaskedTensor is a prototype, the BC breaking nature of this change seems okay?
Locally tested:
<img width="1372" alt="image" src="https://github.com/pytorch/pytorch/assets/31798555/239e61ba-e0b9-4909-8c7a-0ce3869d7375">
Pull Request resolved: https://github.com/pytorch/pytorch/pull/110725
Approved by: https://github.com/cpuhrsch
