[fsdp] fix: Fixing the error caused by empty tensors in the multi_turn + remove_padding scenario

[nuerxiati]

fix the BUG caused by empty tensors in the multi_turn + remove_padding scenario

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In multi-turn scenarios, the attention_mask may encounter full masking. When use_remove_padding is enabled and mbs is sufficiently small, the forward input_ids and position_ids may become an empty tensor, triggering an error.

This PR creates an input containing at least one element during full masking and removes it from the output, preventing this error and enhancing code robustness.

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I used qwen 3b with mbs set to 1, and compared the reward between enabling use_remove_padding (the branch will follow the modified code) and not enabling it.

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[nuerxiati]

@FightingZhen @ji-huazhong，please help me review

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