[Kernel] Correctly invoke prefill & decode kernels for cross-attention (towards eventual encoder/decoder model support)

[afeldman-nm]

This PR is a step towards encoder/decoder model support. This PR modifies the xFormers backend* such that (1) the attention impl can implement cross-attention, and (2) the attention metadata data structure can represent the necessary metadata for invoking cross-attention.

* FlashAttention backend support for encoder/decoder models is left as future work

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A quick overview of the plan for supporting encoder/decoder models in vLLM:

• Architectural assumptions:
  • The encoder/decoder model comprises one non-autoregressive encoder module and one autoregressive decoder module.
  • A single inference call to the model consumes an encoder prompt and a decoder prompt. The model output is the result of decoder inference against the decoder prompt, conditional on the encoder hidden states which result from applying the encoder to the encoder prompt. The encoder hidden states are not part of the overall model output
  • Thus, encoder inference is a prerequisite for decoder inference. The decoder consumes encoder hidden states via cross-attention, which is not present in decoder-only models.
  • It is assumed that these architectural details are handled inside the model definition; however, to support the inference process for such models, vLLM core must be changed to accommodate cross-attention
• Encoder/decoder inference process & cross-attention:

  • Prefill phase: (1) Non-autoregressive encoder inference yields encoder hidden states in a single pass; no KV caching occurs. (2) decoder prefill yields first-token-prediction & cached KVs. Within the decoder, cross-attention layers cache the KVs derived from encoder hidden states:

    • Key_{cross-attn, layer-n} = W_{K, cross-attn, layer-n} x (Encoder hidden states)

    • Value_{cross-attn, layer-n} = W_{V, cross-attn, layer-n} x (Encoder hidden states)

    • Note that all cross-attention layers consume the same encoder hidden states; however each cross-attention layers' keys and values differ because each layer has unique W_{K, cross-attn, layer-n} and W_{V, cross-attn, layer-n}. Therefore, the cross-attention KV cache must store KVs for each decoder layer, even though these KVs are all derived from a single set of encoder hidden states.

    • Note that self-attention layer behavior is unchanged compared to what it would be in a decoder-only model (cache KVs computed from the previous decoder layer outputs.)

  • Decode phase: during each iteration of the autoregressive decode process,

    • Each self-attention layer appends the last predicted token's KVs to the KV cache, and then utilizes cached KVs for next-token prediction (again, this is unchanged compared to a decoder-only model)
    • Each cross-attention layer has read-only access to cross-attention KVs, to use for next-token prediction. The cross-attention KV cache is never modified after prefill

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To implement the above encoder/decoder inference process, the following functionality will be added to vLLM over the course of multiple PRs:

1. Support cross-attention KV cache & memory management (allocate/swap/free) in block manager
   • See PR: [Core] Cross-attention KV caching and memory-management (towards eventual encoder/decoder model support) #4837
2. (This PR) Invoke cross-attention operation via the Attention wrapper & Attention metadata data structure
3. Modify ModelRunner to construct input tensors & Attention metadata structures for cross-attention
4. Small changes to LLM engine & scheduler so that vLLM requests can include an encoder input prompt

Note 1: because this PR makes an incremental contribution (cross-attention KV-caching and memory management), this PR will not enable end-to-end encoder/decoder support (this will rely on later PRs.)

Note 2: the best effort is being made to ensure that encoder/decoder models are compatible with existing vLLM features. At this time, encoder/decoder models are unlikely to be compatible with the following vLLM features:

• Speculative decoding
• Chunked prefill
• Automatic prefix caching
• Sliding window
• Flash attention
• CUDA graph

INCREMENTAL FIX TOWARDS #187

BEFORE SUBMITTING, PLEASE READ THE CHECKLIST BELOW AND FILL IN THE DESCRIPTION ABOVE

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