pip install -e ".[dev]" fails on macOS: liger-kernel requires triton

[clementtisseau]

What does this PR do?

The ligerand dev extras list liger-kernel unconditionally. liger-kernel is pure Python, but requires
triton>=2.3.1, and triton ships manylinux wheels only, no sdist, no macOS/Windows wheels. With nothing to install and
nothing to build, the resolver fails before the install phase, so nothing lands at all.
Windows is affected too.

Skipping liger-kernel on macOS costs nothing: Triton kernels can't run there,
is_liger_kernel_available() already guards the runtime path, and require_liger_kernel
already skips the tests.

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Note

Low Risk
Packaging-only change with no runtime code paths modified; Linux installs keep the same dependency set.

Overview
Fixes pip install -e ".[dev]" (and .[liger]) failing on macOS and Windows by adding a sys_platform == 'linux' marker on liger-kernel in both the liger and dev optional dependency groups.

liger-kernel pulls in Triton, which only publishes Linux wheels, so unconditional inclusion made the resolver fail before any packages installed. On non-Linux platforms this matches existing behavior: Liger/Triton kernels are not usable there, and availability checks plus require_liger_kernel already skip runtime and test paths when the package is absent.

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