Add macros and helper classes in event_tracer needed for profiling (#294)

Summary:
Pull Request resolved: https://github.com/pytorch/executorch/pull/294

All the internal invocations (inside ExecuTorch codebase) of event_tracer will be controlled through the macros added in this diff. The benefit of this is that we can leave these hooks in the hot path and when the flag is disabled they essentially compile to a no-op in prod releases.

It's also ensured that in all these macros if we pass in a `nullptr` as event_tracer (default case if user doesn't pass in one when method is created) we just return immediately.

The specific macros added here are only meant to be used inside the ExecuTorch codebase as the header explains.

Reviewed By: dbort

Differential Revision: D49204729

fbshipit-source-id: 9bd2945ad94a5064aab6076210346b14ed9bcf11