[attention][DCP] use AttentionImpl.need_to_return_lse_for_decode

[youkaichao]

Purpose

Expose a new attribute to avoid hardcoded conditions.

Might be cleaner than #24369 ?

Test Plan

Test Result

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[youkaichao]

hope the error message is clear enough:

DCP requires attention impls to return the softmax lse for decode, but the impl TritonMLAImpl does not return the softmax lse for decode.