add ssd IO/mem stats monitoring

Summary:
add following metrics

1. rocksdb dram mem stats
2. rocksdb io performance duration
3. prefetch duration

Differential Revision: D60413462

torchrec/distributed/tests/test_model_parallel_nccl_ssd_single_gpu.py

@@ -70,6 +70,7 @@ def _load_split_embedding_weights(
             weight,
             torch.as_tensor([height]),
             table_id,
+            False,  # is_bwd
         )
         cum_sum += height