Replace global class-variable track ID counters with per-instance allocators

[cdeil]

What does this PR do?

Fix duplicate track IDs when multiple tracker instances of the same type are used in one process.

Track IDs are the labels returned in detections.tracker_id, for example person A gets tracker_id=0, person B gets tracker_id=1, and the next new track should get tracker_id=2. Previously, that "next ID" counter lived on the internal tracklet class, so all SORTTracker() instances shared it.

That meant resetting one tracker could accidentally affect another live tracker:

tracker_a = SORTTracker()
tracker_b = SORTTracker()

# tracker_a creates a live track with ID 0
tracker_a.update(detections_for_person_a)

# tracker_b resets the shared SORTTracklet counter back to 0
tracker_b.reset()

# tracker_a later creates another live track
# before this PR, that second live track could also get ID 0
tracker_a.update(detections_for_person_a_and_b)

This PR moves the counter to each tracker instance. tracker_b.reset() still restarts IDs from zero for tracker_b, but it no longer changes the IDs that tracker_a will allocate.

This can affect normal scripts and notebooks, not only multi-threaded applications. It is enough to have two trackers of the same type alive at once, for example in multi-camera tracking, class-specific tracking, side-by-side config comparisons, services handling multiple requests, or notebook experiments.

Type of Change

I think this is a bug fix with a small internal breaking change: BaseTracklet.count_id / Tracklet.get_next_tracker_id() were removed because tracklets no longer own ID allocation.

Testing

• I have tested this change locally
• I have added/updated tests for this change

Red/green TDD:

• Added test_tracker_instances_do_not_share_id_allocators, which failed for SORT, ByteTrack, OC-SORT, and BoT-SORT with duplicate live IDs [0, 0].
• Applied the per-instance allocator fix and reran the regression plus existing reset/tracklet contracts successfully.
• Ran the full pytest suite and ruff checks successfully.

Checklist

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code where necessary, particularly in hard-to-understand areas
• My changes generate no new warnings or errors
• I have updated the documentation accordingly (if applicable)

[Copilot]

Pull request overview

This PR fixes duplicate track IDs when multiple tracker instances of the same type coexist by moving track ID allocation from global (tracklet class-level) counters to per-tracker-instance allocators.

Changes:

• Removed BaseTracklet.count_id / get_next_tracker_id() and all tracklet subclass counters.
• Added per-instance ID allocation helpers to BaseTracker, and updated SORT/ByteTrack/OC-SORT/BoT-SORT to use them (including on reset()).
• Updated tests to cover the “two live instances must not share/reset each other’s IDs” regression, and removed the now-obsolete class-counter test.

Reviewed changes

Copilot reviewed 13 out of 13 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
tests/core/test_tracklets.py	Removes the test that asserted per-tracklet-subclass global counters (no longer applicable).
tests/core/test_trackers.py	Adds a regression test ensuring two tracker instances don’t share ID allocation; adds a small detections helper.
src/trackers/utils/base_tracklet.py	Removes the global class-variable ID counter and allocator from the base tracklet.
src/trackers/core/sort/tracklet.py	Removes SORT tracklet’s class-level ID counter.
src/trackers/core/sort/tracker.py	Switches SORT to per-instance ID allocation and resets allocator on init/reset.
src/trackers/core/ocsort/tracklet.py	Removes class-level counter and threads an allocator callable into resolve_tracker_id().
src/trackers/core/ocsort/tracker.py	Uses per-instance allocator and passes it into resolve_tracker_id(); resets allocator on init/reset.
src/trackers/core/bytetrack/tracklet.py	Removes ByteTrack tracklet’s class-level ID counter.
src/trackers/core/bytetrack/tracker.py	Switches ByteTrack to per-instance ID allocation and resets allocator on init/reset.
src/trackers/core/botsort/tracklet.py	Removes BoT-SORT tracklet’s class-level ID counter.
src/trackers/core/botsort/tracker.py	Switches BoT-SORT to per-instance ID allocation and resets allocator on init/reset.
src/trackers/core/base.py	Adds _reset_id_allocator() / _allocate_tracker_id() helpers to BaseTracker.
CHANGELOG.md	Documents the internal breaking change and the user-facing fix.

[AlexBodner]

@cdeil thanks for this PR!. @Borda this is something i wanted to do some time ago, so its great. the only problem is that it will generate conflicts with CBIou and mcbyte. So I would first merge those PRs, then extend this one to cover those trackers and finally merging this PR