Give each host a unique rank in Hostfile.from_list

[ayaangazali]

Proposed changes

Hostfile.from_list builds every Host with the wrong rank. The inner repeats loop reuses the name of the enumerate index:

for i, h in enumerate(hostlist.split(",")):
    ...
    for i in range(repeats):        # shadows the enumerate index
        hosts.append(Host(i, h, ips, []))

so the value passed to Host is the repeat counter, never the position in the list. With the default repeats=1 the range is always range(1), so every host gets rank 0.

from_file, the sibling constructor, assigns Host(i, ...) straight from enumerate, so a unique sequential rank is clearly the intent.

Repro

from mlx._distributed_utils.common import Hostfile

[(h.rank, h.ssh_hostname) for h in Hostfile.from_list("hostA,hostB,hostC").hosts]
# before: [(0, 'hostA'), (0, 'hostB'), (0, 'hostC')]
# after:  [(0, 'hostA'), (1, 'hostB'), (2, 'hostC')]

[(h.rank, h.ssh_hostname) for h in Hostfile.from_list("hostA,hostB", 3).hosts]
# before: [(0, 'hostA'), (1, 'hostA'), (2, 'hostA'), (0, 'hostB'), (1, 'hostB'), (2, 'hostB')]
# after:  [(0, 'hostA'), (1, 'hostA'), (2, 'hostA'), (3, 'hostB'), (4, 'hostB'), (5, 'hostB')]

Using the running length of the list keeps ranks unique across repeats too.

Scope

To be upfront: nothing reads Host.rank today, so this is latent rather than a live failure — I found it while reading the launcher, not from a bug report. It is still a wrong value on a public dataclass field that from_file populates correctly, so it seemed worth correcting before something starts depending on it. Close it if you would rather leave the field alone.

Behaviour that is unchanged: empty hostnames still raise, and literal IPs are still detected and stored in ips.

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)