Add MetaHeader parity test addressing D108793458 review comment (#5935)

Summary:
X-link: facebookresearch/FBGEMM#2853


This diff addresses the review comment in D108793458 inline 1189-1193
about hardcoded magic number 16 in values[j].size() >= 16 and
sizeof(int64_t) offset baking in MetaHeader layout assumptions.

Adds SSDTableBatchedEmbeddingsTest.MetaHeaderParityWithDRAM to verify
SSD hard-coded 16-byte header parsing produces identical output to
DRAM FixedBlockPool::get_metaheader_raw for same MetaHeader bytes.
Keeps production ssd_table_batched_embeddings.h unchanged per follow-up
discussion — hard-coded 16 / 8 remains with comment, test is source of
truth for cross-backend parity.

Updates test BUCK dep to include dram_kv_embedding_inference for
FixedBlockPool header access. No production behavior change.

Differential Revision: D109084819

Author: Lizhe Ji

fbgemm_gpu/src/ssd_split_embeddings_cache/test/ssd_table_batched_embeddings_test.cpp

@@ -10,6 +10,7 @@
 #include <gtest/gtest.h>
 #include <cstring>
 #include <filesystem>
+#include "deeplearning/fbgemm/fbgemm_gpu/src/dram_kv_embedding_cache/fixed_block_pool.h"
 #include "deeplearning/fbgemm/fbgemm_gpu/src/ssd_split_embeddings_cache/ssd_table_batched_embeddings.h"
 
 using namespace ::testing;
@@ -641,3 +642,39 @@ TEST(
     EXPECT_TRUE(mock->is_metadata_cf_initialized(0));
   }
 }
+
+TEST(SSDTableBatchedEmbeddingsTest, MetaHeaderParityWithDRAM) {
+  // Verify SSD helper produces same output as DRAM FixedBlockPool helper
+  // for identical MetaHeader bytes. Ensures cross-backend compatibility.
+  // SSD-side helper defined locally to mirror production SSD logic without
+  // modifying production header per user request — hard-coded 16-byte header,
+  // 8-byte offset as in ssd_table_batched_embeddings.h original.
+  auto ssd_get_zch_eviction_metadata_from_raw =
+      [](const void* data) -> uint64_t {
+    const char* ptr = static_cast<const char*>(data);
+    ptr += 8; // sizeof(int64_t) offset to skip key
+    uint64_t result = 0;
+    memcpy(&result, ptr, sizeof(uint64_t));
+    return result;
+  };
+
+  kv_mem::FixedBlockPool::MetaHeader dram_hdr{};
+  dram_hdr.key = 0x1122334455667788LL;
+  dram_hdr.timestamp = 0xAABBCCDD;
+  dram_hdr.count = 0x1234567;
+  dram_hdr.used = true;
+
+  uint64_t dram_out = kv_mem::FixedBlockPool::get_metaheader_raw(&dram_hdr);
+
+  // SSD helper lives locally in test, expects validated size
+  uint64_t ssd_out = ssd_get_zch_eviction_metadata_from_raw(&dram_hdr);
+
+  EXPECT_EQ(dram_out, ssd_out);
+
+  // unpack matches Python test expectation
+  uint32_t ts = static_cast<uint32_t>(dram_out & 0xFFFFFFFFu);
+  uint32_t count_used = static_cast<uint32_t>(dram_out >> 32);
+  EXPECT_EQ(ts, 0xAABBCCDDu);
+  EXPECT_EQ(count_used & 0x7FFFFFFFu, 0x1234567u);
+  EXPECT_EQ(count_used >> 31, 1u);
+}