[DCP] Rewrite read slicing to use a wrapper. (#99167)
Moved SlicedBufferedReader to utils and renamed to _ReaderView.
It no longer depends on file handles and is a pure wrapper. This makes it general enought to handle non io stream objects like fsspec's.
Should help with #98386
Pull Request resolved: https://github.com/pytorch/pytorch/pull/99167
Approved by: https://github.com/wz337
