Set kernels optional dependency via transformers

[albertvillanova]

Set kernels optional dependency via transformers[kernels].

This PR updates the way the kernels dependency is specified in both the main and development dependencies in pyproject.toml. Instead of directly depending on the kernels package, the project now relies on the transformers[kernels] extra, which should simplify dependency management and improve compatibility.

Related to:

• CI is broken: ValueError: Either a revision or a version must be specified #5878
• Fix ValueError by pinning kernels < 0.15.1 #5880

Motivation

Since TRL does not directly depend on the kernels package itself, the new extra simply re-exports the corresponding transformers extra instead of duplicating its dependency specification. This keeps transformers as the single source of truth for the underlying package requirements and version constraints.

This also ensures we automatically inherit both the lower and upper version bounds defined upstream (and any future updates to them), avoiding duplication in TRL.

Changes

Dependency management updates:

• Replaced the direct dependency on kernels<0.15.1 with transformers[kernels] in the main dependencies list.
• Updated the development dependencies to use transformers[kernels] instead of kernels<0.15.1.

---

Note

Low Risk
Packaging-only change with no runtime code; install graphs may shift if Transformers' kernels extra differs from the old pin.

Overview
The kernels optional extra and the dev dependency bundle no longer pin kernels<0.15.1 directly. Both now install transformers[kernels], so kernel package versions and bounds come from Transformers instead of being duplicated in TRL (related to GH-5878 / #5880).

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