[megatron] integrate MCore blackwell linear_cross_entropy fusion kernel

[HaochenYuan]

What does this PR do?

• Integrate fused lm_head & cross-entropy kernel from megatron link. Currently only support blackwell.
• Add GB300/B300 flops data.

The test is done on B300 with Qwen3-30B-A3B.

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[ISEEKYAN]

Hi @HaochenYuan, I would like to remove the verl/models/mcore/model_forward_fused.py and make this fused kernel called inside mcore, so we don't need to maintain another forward_function as a entrance, the only need is to add a transformer_config to enable this kernel.
But existing kernel only supports Blackwell, it will be necessary to Hopper for RL workloads now. NVIDIA/Megatron-LM#3076