[clad] Bump to v2.1

[vgvassilev]

Clad 2.1 introduces major advancements in reverse mode differentiation, bringing smarter handling of loops, assignments, and method calls, alongside the new clad::restore_tracker for functions that modify their inputs. Forward mode gains static scheduling for Hessians and higher-order derivatives, while CUDA support expands with custom derivatives for key Thrust algorithms such as reduce, transform, and transform_reduce, plus optimizations that reduce unnecessary GPU atomics. The release also strengthens error estimation, simplifies adjoint initialization, improves tape efficiency, and enhances diagnostics. With a migration to C++17, support extended up to clang-21, and numerous bug fixes, Clad 2.1 delivers faster, safer, and more reliable automatic differentiation across CPU and GPU workflows.

[vgvassilev]

@guitargeek, can you take a look at the failures and interpret them for us? IIUC, there is nothing clad related...

[github-actions]

Test Results

    22 files      22 suites   3d 20h 12m 8s ⏱️
 3 680 tests  3 680 ✅ 0 💤 0 ❌
79 071 runs  79 071 ✅ 0 💤 0 ❌

Results for commit 122d0d0.

♻️ This comment has been updated with latest results.

[hahnjo]

You probably want a clean build because there may be files left over when just changing the GIT_TAG version

[guitargeek]

@guitargeek, can you take a look at the failures and interpret them for us? IIUC, there is nothing clad related...

There are comparison failures with numeric gradients now that fail all over the place, so the current failures seem Clad related.

I'll have time to investigate this later in the afternoon after the ROOT meeting. I can create a reproducer and open an issue in Clad a usual.

[vgvassilev]

Thank you, @guitargeek!

cc: @PetroZarytskyi

[guitargeek]

I'm checking now Clad master with Petros possible fix on top:

• Test out Clad master fix from Petro #19961

[guitargeek]

Note that while the ROOT CI will pass now, there is still a regression in Clad master that broke the CMS Combine likelihoods:

• Regression in Clad master when aliasing array parameters in inner function vgvassilev/clad#1573

[vgvassilev]

Note that while the ROOT CI will pass now, there is still a regression in Clad master that broke the CMS Combine likelihoods:

* [Regression in Clad `master` when aliasing array parameters in inner function vgvassilev/clad#1573](https://github.com/vgvassilev/clad/issues/1573)

Ah, okay, thanks for clarifying.

[guitargeek]

Thank you very much!