[DataPipe] Add container template for _Fork and _Demux (#89216)
- This would remove the hard-coded check within `_ChildDataPipe`.
- Add `get_length_by_instance` to parent class to make sure there is a chance that child DataPipe can have different lengths
- Prevent Error when `__del__` executed when the object has already been removed
Pull Request resolved: https://github.com/pytorch/pytorch/pull/89216
Approved by: https://github.com/NivekT
