Remove unexpected kwargs passing to flce

[Tcc0403]

Summary

Resolves #650

**kwargs might contain FlashAttentionKwargs, so we should avoid passing it to flce directly.

This PR also adopts the changes of ForCausalLMLoss and fixed_cross_entropy from huggingface/transformers, and rename softcap to final_logit_softcapping to match the naming.

Testing Done

• Hardware Type:
• run make test to ensure correctness
• run make checkstyle to ensure code style
• run make test-convergence to ensure convergence

[Tcc0403]

Investigating why gemma3 multimodal test failed

@eljandoubi should we relax the tolerance further?

[shivam15s]

overall looks good; don't understand why are we not passing kwargs to liger flce anymore

[Tcc0403]

Mainly because **kwargs might contain FlashAttentionKwargs, we might accidently pass them to flce when users set _attn_implementation to flash_attention-2.

We already catch required args by declaring names explicitly in function signature, so the rest is not needed. There are many features (weight, label_smoothing, z-loss, ...) in flce, and I thought they were good to have. But most of them are not supported in transformers, I just remove them for now in case future changes of transformers breaking it again.

[shivam15s]

Cool, thanks for the explanation

[shivam15s]

lgtm!