[Feature]: Enable MLA prefill-only operator

[nzmora-nvidia]

🚀 The feature, motivation and pitch

FlashInfer MLA MHA-mode (no weight absorption) with cache; ragged; for prefill-only. Uses flashinfer.BatchPrefillWithRaggedKVCacheWrapper

This kernel is optimized for prefill.
Its inputs are full heads Q/K/V so it performs MLA in MHA-mode (no absorption).
The operator should compute ckv and kpe and write them to the compressed cache, then continue to do MHA.
This can probably done with other flashinfer kernels.

Alternatives

No response

Additional context

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