Resolved alibi bias issue due to porting flat PA pr

Signed-off-by: Tanner Voas <tanner.voas@intel.com>

vllm_hpu_extension/ops.py

@@ -137,10 +137,52 @@ def block_softmax(batch_size, attn, block_mapping, block_scales, block_groups):
     return attn
 
 
-def flat_pa(query, key_cache, value_cache, block_list, block_mapping,
-            block_bias, block_scales, block_groups, scale, matmul_qk_op, matmul_av_op, keys_fetch_func,
-            values_fetch_func):
+def process_alibi_biases(alibi_biases, block_list, block_size, batch_size, num_heads):
+    # alibi_biases: [1, num_heads, seq_length, seq_length]
+    # block_list: [num_blocks]
+    # block_size: scalar integer
+    # Assume batch_size and num_heads are inferred from alibi_biases and block_list
+    seq_length = alibi_biases.size(2)
+    num_blocks = block_list.size(0)
+    block_per_sequence = seq_length // block_size + (1 if seq_length % block_size > 0 else 0)
+
+    # Step 1: Reduce alibi_biases to shape [1, num_heads, seq_length]
+    alibi_biases = alibi_biases[:, :, -1:, :].squeeze(-2)  # Shape [1, num_heads, seq_length]
+
+    # Step 2: Expand alibi_biases to shape [batch_size, num_heads, seq_length]
+    alibi_biases = alibi_biases.expand(batch_size, num_heads, seq_length)
+
+    # Step 3: reshape alibi_biases to [batch_size, block_per_sequence, num_heads, block_size]
+    alibi_blocks = alibi_biases.view(batch_size, num_heads, block_per_sequence, block_size)
+    alibi_blocks = alibi_blocks.permute(0, 2, 1, 3)
+    alibi_blocks = alibi_blocks.contiguous()
+    alibi_blocks = alibi_blocks.view(-1, num_heads, block_size)
+
+    # Step 4: Use block_list to index into alibi_blocks
+    output = torch.zeros((num_blocks, num_heads, block_size), device=block_list.device, dtype=alibi_biases.dtype)
+    output[block_list[:alibi_blocks.size(0)]] = alibi_blocks
+    output = output.unsqueeze(-2)
+
+    return output
+
+def flat_pa(
+    query,
+    key_cache,
+    value_cache,
+    block_list,
+    block_mapping,
+    block_bias,
+    block_scales,
+    block_groups,
+    scale,
+    alibi_slopes,
+    matmul_qk_op,
+    matmul_av_op,
+    keys_fetch_func,
+    values_fetch_func,
+):
     batch_size = query.size(0)
+    block_size = key_cache.size(1)
     q_heads = query.size(1)
     kv_heads = key_cache.size(2)
 
@@ -158,6 +200,9 @@ def flat_pa(query, key_cache, value_cache, block_list, block_mapping,
         key = key.transpose(2, 3)
 
     attn = matmul_qk_op(query, key) + block_bias
+    if alibi_slopes is not None:
+        block_alibi_slopes = process_alibi_biases(alibi_slopes, block_list, block_size, batch_size, kv_heads)
+        attn.add_(block_alibi_slopes)
     attn = block_softmax(batch_size, attn, block_mapping, block_scales, block_groups)
     attn = matmul_av_op(attn, value)
     attn = block2batch(attn, block_mapping)