profiler: improve repr for averaged events (#20281)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/20281
This is how it looks like now:
```
<FunctionEventAvg key=mm self_cpu_time=11.404s cpu_time=2.895ms
cuda_time=0.000us input_shapes=[[26, 4096], [4096, 1024]]>
```
Before I forgot to update the repr for these when updated it for not
averaged events
Differential Revision: D15262862
fbshipit-source-id: a9e5b32c347b31118f98b4b5bf2bf46c1cc6d0d2
