Fix crash when reporting partial rings in mlx.distributed_config

[ayaangazali]

Proposed changes

When mlx.distributed_config cannot find a full ring it tries to print the partial rings it did find, but that diagnostic path raises TypeError instead:

TypeError: list indices must be integers or slices, not list

extract_rings returns (path, count) pairs:

rings.append((r, cnt))
...
return sorted(rings, key=lambda x: -len(x[0]))

check_valid_ring already unpacks correctly when testing for a full ring (len(rings[0][0]) == len(hosts)), but the error branch iterates the pairs as if each element were a bare path and indexes hosts with it:

for r in rings:
    log_error(f" - {','.join(hosts[i].ssh_hostname for i in r)}")

so i is the path list itself rather than a node index.

This means the failure is worst exactly when the diagnostic matters most: a user whose fabric is miswired gets a traceback instead of the list of rings that were found, and loses the hint to run --dot.

Repro

The branch is on the error path whenever a partial ring exists and no full ring does:

class H:
    def __init__(self, n): self.ssh_hostname = n

hosts = [H("h1"), H("h2"), H("h3")]
rings = [([0, 1], 2)]                     # partial ring, shape as returned by extract_rings

for r in rings:                           # current code
    print(" - " + ",".join(hosts[i].ssh_hostname for i in r))
# TypeError: list indices must be integers or slices, not list

After the change the same input prints what was intended:

 - h1,h2

Checklist

• I have read the CONTRIBUTING document
• I have run pre-commit run --all-files to format my code / installed pre-commit prior to committing changes
• I have added tests that prove my fix is effective or that my feature works
• I have updated the necessary documentation (if needed)

No test added: this is a one-line fix in an error-reporting branch that needs a miswired multi-host Thunderbolt fabric to reach naturally, and this repo has trimmed tests from small fixes of this size before (#3914). Happy to add one if you would prefer.