Add implementation for irregular dimension selection for nested tensors. (#88585)
Summary: This diff modifies the implementation of the select operator so slices of the irregular dimension can be selected (e.g. nt[:,0,:]).
Test Plan:
Added new unit tests to test that the new functions work as intended (see them in diff). To test,
`buck test mode/dev-nosan //caffe2/test:nested`
Differential Revision: D41083993
Pull Request resolved: https://github.com/pytorch/pytorch/pull/88585
Approved by: https://github.com/cpuhrsch
Author
Antoni Viros i Martin
