[Profiler] Introduce trace_me decorator (#4809)
Summary:
This pull request introduces a new profiler API called trace_me decorator. I find it very tedious if I want to scope a method as I need to first use the xp.Trace("") context manager and then indent the whole method. Now, you can just do:
```
import torch_xla.debug.profiler as xp
@xp.trace_me("forward")
def forward:
...
```
Test Plan:
PJRT_DEVICE=CPU python test/test_operations.py -v -k XpTraceTest
