[Feature] Split cross-entropy computation in SP

[Edenzzzz]

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📝 What does this PR do?

1. Avoid gathering hidden states before cross entropy computation to save comm & compute. All-reduce the final loss & grad instead.
2. Support SP for GPT
3. Merge the logic of PP model fwd and model fwd for GPT2 and Llama for easy maintainence in the future.

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