[Refactor] Decouple topk and loss from DSA Indexer

[laixinn]

What does this PR do ?

After this refactor, DSA is compatible with the upcoming fused kernel.

PR for main #3248

Unit Test

• tests/unit_tests/transformer/test_attention_variant_dsa.py

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[duncanriach]

All clear for hybrid-mamba

[kunlunl]

/ok to test a857d6a

[kunlunl]

/ok to test 5f6769e

[kunlunl]

/ok to test d3adddf