Fix Subset of a Subset not sliceable issue (#59513)
Summary:
Dataset can be indexed by a list, but a list can not be indexed by a list. This gives error when slicing a Subset initialised with a Subset, instead of a dataset.
Fixed the issue by changing the indices to a Tensor which can be indexed by a list.
Fixes https://github.com/pytorch/pytorch/issues/59512
Pull Request resolved: https://github.com/pytorch/pytorch/pull/59513
Reviewed By: zou3519
Differential Revision: D29196891
Pulled By: ejguan
fbshipit-source-id: ccde6e474fbcbddd2e9c7c107bc8b5de1307cdb9
