[kineto] init kineto_activity for each event (#97550)
Summary:
There was a refactoring while back to address Kineto <--> PyTorch Profiler buffer management issues. This made the Profiler API path safer but it regressed the OnDemand path.
The proper long term solution is to merge those paths which would significantly improve the maintainability of the codebase.
Test Plan:
# Test on Resnet integration test
```
buck2 run mode/opt kineto/libkineto/fb/integration_tests:pytorch_resnet_integration_test
dyno gputrace
```
# Trace
https://fburl.com/perfdoctor/t8nkda9z
Differential Revision: D44362040
Pull Request resolved: https://github.com/pytorch/pytorch/pull/97550
Approved by: https://github.com/aaronenyeshi
