[torch][segment_reduce] Add support for initial value (#56923)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/56923
Next Steps in order:
- Add backward support for CUDA
- Add support for more aggregation types
- Benchmarking (for cuda mainly)/more testing/documentation
- Support for multi dimension
Test Plan: Updated unit test to include 0 length segment as well.
Reviewed By: ngimel
Differential Revision: D27992228
fbshipit-source-id: 28851811f8a784a63162721c511d69e617a93727
