[profiler] add a function to allow adding preset user-defined metadata to traces (#121487)
Summary:
`add_metadata_json` function in profiler can only work when being called during trace collection. However, sometimes we want to pass in some user-defined metadata and amend to the trace before trace collection starts, e.g. when the profiler is defined.
This PR add a function `preset_metadata_json` for this purpose. The preset metadata will be stored and amended to the trace later.
Differential Revision: D54678790
Pull Request resolved: https://github.com/pytorch/pytorch/pull/121487
Approved by: https://github.com/aaronenyeshi
