fix: Only bind device id when needed, Fixes #8248

[pengdurice]

Fixes #8248.

The change

init_process_group now asks a small resolver instead of inlining the condition:

device_id = get_init_process_group_device_id(world_size)
if device_id is not None:
    kwargs.update(device_id=device_id)

DEEPSPEED_SET_DEVICE_ID overrides the decision, parsed by a new get_env_flag helper:

Value	Effect
1 / true / yes / on	always bind
0 / false / no / off	never bind
unset	the default above: bind iff world_size > 1
anything else	warn and ignore, so the default still decides

Multi-rank behaviour is unchanged.

Tests

tests/unit/comm/test_dist.py

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