[Bugfix] Fix index out of bounds in local attention with large sequences (sgl-project#5173)

Author: CatherineSue

python/sglang/srt/layers/attention/flashattention_backend.py

@@ -236,7 +236,11 @@ def make_local_attention_virtual_batches(
         np.arange(pages_per_local_batch, dtype=np.int32),
         (virtual_batches, pages_per_local_batch),
     ) + np.expand_dims(block_starts, axis=1)
-    block_indices = block_indices.flatten()
+    # Ensure block_indices doesn't exceed block_table dimensions
+    # This is a critical safety check that prevents index out of bounds errors
+    # when dealing with large sequences (>8192 tokens) or when the block_table
+    # dimensions are smaller than what would be needed for the full attention chunk size.
+    block_indices = block_indices.flatten().clip(max=block_table.shape[1] - 1)
     batch_indices = np.repeat(
         np.arange(actual_batch_size, dtype=np.int32),
         local_blocks * pages_per_local_batch,