[PyTorch] Optimize no input NVTX collection (pytorch#70133)

Summary:
Pull Request resolved: pytorch#70133

we were creating `sstream` + string concats via `getNvtxStr` even when there were no inputs and wasting precious time. this diff avoids `stringstream` when there is no input to squeeze performance. 60% reduction in overhead

Test Plan:
Before
```
I1214 22:48:07.964118 2971180 bench.cpp:154] Mean 0.970494
I1214 22:48:07.964139 2971180 bench.cpp:155] Median 0.969054
I1214 22:48:07.964144 2971180 bench.cpp:156] Min 0.962247
I1214 22:48:07.964148 2971180 bench.cpp:157] stddev 0.00774841
I1214 22:48:07.964154 2971180 bench.cpp:158] stddev / mean 0.00798398
```

After
```
I1214 22:59:00.039872 3437853 bench.cpp:154] Mean 0.384333
I1214 22:59:00.039896 3437853 bench.cpp:155] Median 0.384886
I1214 22:59:00.039899 3437853 bench.cpp:156] Min 0.370235
I1214 22:59:00.039902 3437853 bench.cpp:157] stddev 0.00435907
I1214 22:59:00.039907 3437853 bench.cpp:158] stddev / mean 0.0113419
```

Reviewed By: aaronenyeshi, robieta

Differential Revision: D33137501

fbshipit-source-id: ce0e8cf9aef7ea22fd8aed927e76be4ca375efc3

torch/csrc/profiler/util.cpp

@@ -1,6 +1,7 @@
 #include <torch/csrc/profiler/util.h>
 
 #include <c10/util/ArrayRef.h>
+#include <fmt/format.h>
 
 #ifdef USE_KINETO
 #include <libkineto.h>
@@ -31,22 +32,20 @@ std::string getNvtxStr(
     int64_t sequence_nr,
     const std::vector<std::vector<int64_t>>& shapes) {
   if (sequence_nr >= -1 || shapes.size() > 0) {
-    std::stringstream s;
-#if defined(USE_ROCM)
-    s << name;
-#endif
+    std::string str;
     if (sequence_nr >= 0) {
-#if defined(USE_ROCM)
-      s << ", seq = " << sequence_nr;
-#else
-      s << name << ", seq = " << sequence_nr;
-#endif
+      str = fmt::format("{}, seq = {}", name, sequence_nr);
     } else if (sequence_nr == -1) {
-#if !defined(USE_ROCM)
-      s << name;
+      str = name;
+    } else {
+#if defined(USE_ROCM)
+      // Only ROCM supports < -1 sequence_nr
+      str = name;
 #endif
     }
     if (shapes.size() > 0) {
+      std::stringstream s;
+      s << str;
       s << ", sizes = [";
       for (const auto idx : c10::irange(shapes.size())) {
         if (shapes[idx].size() > 0) {
@@ -66,8 +65,10 @@ std::string getNvtxStr(
         }
       }
       s << "]";
+      return s.str();
     }
-    return s.str();
+
+    return str;
   } else {
     return name;
   }