add SP support for _flash_3_varlen_hub backend

[zhtmike]

What does this PR do?

A follow up work for #13479. I have added _flash_3_varlen_hub support for SP forward & backward.

Tested with QwenImage pipeline, the result image is expected.
Tested with QwenImage training with SP, there is no error.
The UTs for Flux and QwenImage are passed.

Fixes # (issue)

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Who can review?

@sayakpaul

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