ci: add pytorch 2.12 and 2.13 to cpu tests

[bhimrazy]

What does this PR do?

Adds PyTorch 2.12 and 2.13 to the CPU test matrices for fabric and pytorch, on Python 3.12 and 3.13 — extending what #21679 did for 2.11.

One test needed fixing to pass on the new versions — test_get_default_process_group_backend_for_device:

• Cause: it called torch.utils.rename_privateuse1_backend("pcu"), which is process-global and irreversible, leaking a dangling pcu device into every later test in the same xdist worker.
• Why it only shows up now: on 2.12/2.13 the tensor-moving paths consult the privateuse1 backend, so unrelated CPU-only tests started failing with RuntimeError: Device 'pcu' does not have a corresponding module registered as 'torch.pcu'.
• Fix: used monkeypatch.setitem on Backend.default_device_backend_map instead, which has no global side effects.

The other blocker here was torch.compile + self.log() crashing on 2.12/2.13; that was split out and merged separately in #21837.

Follow-up

Both matrices now carry a TODO about pruning the oldest minors (2.1–2.5), best done in its own PR.

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Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 87%. Comparing base (35c8970) to head (2a4b3ce).
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Additional details and impacted files

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##           master   #21835   +/-   ##
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  Coverage      87%      87%           
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  Files         270      270           
  Lines       24074    24074           
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  Hits        20903    20903           
  Misses       3171     3171           

[bhimrazy]

requires: #21837