Fix XProf GCS upload bug in max_utils.Profiler

[eltsai]

Creating this PR based on CL:

This change fixes a bug where XProf traces were not uploaded to GCS when the profiler was used manually via start() and stop() calls, which is the case in most MaxDiffusion trainers.

The GCS upload logic was previously located in Profiler.__exit__ (used by the context manager), but not in Profiler.stop(). This change moves the upload logic to stop() and guards it with _jax_profiler_enabled(self.config) to ensure it only runs when JAX profiling was active.

Also:

• Added unit tests in profiler_test.py to verify GCS upload for both manual and context manager usage.
• Added profiler_test target to BUILD.

[github-actions]

e2e testgrid: https://8bcf50593faf4ea38060e236169827e5-dot-us-central1.composer.googleusercontent.com/dags/maxdiffusion_tpu_e2e/grid

[eltsai]

Also disabled profiler for the first two runs. The xprof functions is as expected now:

gcloud storage ls gs://elisatsai-wan-maxdiffusion/wan22/8tpu/ulysses-custom-test-xprof3/ulysses-custom-test-xprof3/tensorboard/plugins/profile/2026_05_15_22_27_45/t1v-n-cd58a6ba-w-0.xplane.pb

gs://elisatsai-wan-maxdiffusion/wan22/8tpu/ulysses-custom-test-xprof3/ulysses-custom-test-xprof3/tensorboard/plugins/profile/2026_05_15_22_27_45/t1v-n-cd58a6ba-w-0.xplane.pb