Add hooks in event_tracer for logging evalues (#1275)

Summary:
Pull Request resolved: https://github.com/pytorch/executorch/pull/1275

This diff primarily adds a hook to event_tracer that enables the logging of EValue's. This will be used for debugging purposes for intermediate tensor logging and model output logging.

In order to enable this, this diff has the following changes:
- Changes to even_tracer.h to add the hook
- Split out the event_tracer target from the core target into a separate event_tracer target as it now needs access to the EValue header.

Reviewed By: dbort

Differential Revision: D51522111

fbshipit-source-id: a234180a0641146574c2dbcf225ba0d9905ae036