[NPU] fix allocator min chunk size (#31632)

Author: zhiqiu

paddle/fluid/platform/npu_info.cc

@@ -163,8 +163,11 @@ size_t NPUInitAllocSize() { return NPUAllocSize(/* realloc = */ false); }
 size_t NPUReallocSize() { return NPUAllocSize(/* realloc = */ true); }
 
 size_t NPUMinChunkSize() {
-  // Allow to allocate the minimum chunk size is 256 bytes.
-  return 1 << 8;
+  // NOTE(zhiqiu): It seems the min chunk size should be 512 on NPU,
+  // though no document specify that explicitly.
+  // See https://gitee.com/zhiqiuchen/Ascend/tree/master/test_reduce_sum_d for
+  // details.
+  return 1 << 9;
 }
 
 size_t NPUMaxChunkSize() {