microgpt 0.5.0: reduce log verbosity, bump version

[aaylward]

Summary

• Reduce training log verbosity: status is now printed every 500 steps instead of every step (~400x less output, saves ~100 MB of terminal scrollback on a 200K step run)
• Extract log_step() helper to clean up the training loop
• Bump microgpt_cli version to 0.5.0

Test plan

• bazel build //... passes
• bazel test //... passes (75/75)

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