Add logcumsumexp forward-ad (#100629)
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### <samp>🤖 Generated by Copilot at 8bb6158</samp>
This pull request adds forward and backward AD support for the `logcumsumexp` operator in functorch, a library for composable function transformations. It implements a forward-mode formula and a decomposition in `derivatives.yaml`, a C++ function for computing directional derivatives in `FunctionsManual.cpp`, and updates the tests and metadata in `test_ops.py` and `common_methods_invocations.py`.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/100629
Approved by: https://github.com/soulitzer
