Add basic support for sparse meta tensors (#81800)
Coverage is by no means complete, we'll drive more coverage using
an appropriate cross-ref tests; this is just enough to get construction
and querying working.
Signed-off-by: Edward Z. Yang <ezyang@fb.com>
Pull Request resolved: https://github.com/pytorch/pytorch/pull/81800
Approved by: https://github.com/cpuhrsch, https://github.com/bdhirsh
