Reject use_liger_kernel in GMPOTrainer

[YeonwooSung]

What does this PR do?

GMPOTrainer only overrides _compute_loss. Inherited GRPOTrainer.compute_loss still routes use_liger_kernel=True through the GRPO fused loss, so training silently skips the geometric-mean objective.

This raises in GMPOTrainer.__init__ before super().__init__, so the Liger path never starts. A Liger GMPO kernel is not added. gmpo.md notes that the inherited flag is unsupported.

Adds test_init_fails_with_liger_kernel.

Fixes #6808

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Note

Low Risk
Small guard at trainer init plus docs and a unit test; no change to the GMPO loss path when use_liger_kernel is false (default).

Overview
GMPOTrainer now fails fast when use_liger_kernel=True, because the inherited GRPO/Liger training path would optimize the GRPO fused loss instead of GMPO’s geometric-mean objective.

A ValueError is raised in GMPOTrainer.__init__ before GRPOTrainer.__init__, so the Liger path never starts. gmpo.md documents that use_liger_kernel is inherited from GRPOConfig but unsupported. test_init_fails_with_liger_kernel asserts construction raises with the expected message.

Fixes #6808.

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