added check to avoid div 0 errors in cache report (pytorch#1645)

Summary:
Pull Request resolved: pytorch#1645

as in title

Differential Revision: D44096435

fbshipit-source-id: c5937aa78b3b16fbd86b2383fe51b93845505135

fbgemm_gpu/fbgemm_gpu/split_table_batched_embeddings_ops.py

@@ -979,10 +979,11 @@ def print_uvm_cache_stats(self) -> None:
             f"N_conflict_unique_misses: {uvm_cache_stats[4]}\n"
             f"N_conflict_misses: {uvm_cache_stats[5]}\n"
         )
-        logging.info(
-            f"unique indices / requested indices: {uvm_cache_stats[2]/uvm_cache_stats[1]}\n"
-            f"unique misses / requested indices: {uvm_cache_stats[3]/uvm_cache_stats[1]}\n"
-        )
+        if uvm_cache_stats[1]:
+            logging.info(
+                f"unique indices / requested indices: {uvm_cache_stats[2]/uvm_cache_stats[1]}\n"
+                f"unique misses / requested indices: {uvm_cache_stats[3]/uvm_cache_stats[1]}\n"
+            )
 
     def prefetch(self, indices: Tensor, offsets: Tensor) -> None:
         self.timestep += 1
@@ -2347,10 +2348,11 @@ def print_uvm_cache_stats(self) -> None:
             f"N_conflict_unique_misses: {uvm_cache_stats[4]}\n"
             f"N_conflict_misses: {uvm_cache_stats[5]}\n"
         )
-        logging.info(
-            f"unique indices / requested indices: {uvm_cache_stats[2]/uvm_cache_stats[1]}\n"
-            f"unique misses / requested indices: {uvm_cache_stats[3]/uvm_cache_stats[1]}\n"
-        )
+        if uvm_cache_stats[1]:
+            logging.info(
+                f"unique indices / requested indices: {uvm_cache_stats[2]/uvm_cache_stats[1]}\n"
+                f"unique misses / requested indices: {uvm_cache_stats[3]/uvm_cache_stats[1]}\n"
+            )
 
     @torch.jit.export
     def prefetch(self, indices: Tensor, offsets: Tensor) -> None: