Validate warmup_type in WarmupCosineLR like WarmupLR

[sohumt123]

Problem

WarmupLR.__init__ validates warmup_type and falls back to log with a warning for unknown values, and WarmupDecayLR inherits that behavior. WarmupCosineLR documents the same {'log', 'linear'} contract but stores warmup_type unvalidated, and get_lr_ratio()'s warmup branch only assigns ratio for the two known types. Any other value (e.g. a typo like 'Linear' or 'cosine') crashes on the first step():

File "deepspeed/runtime/lr_schedules.py", line 850, in get_lr_ratio
    ratio = self.warmup_min_ratio + ratio * ratio_delta
UnboundLocalError: cannot access local variable 'ratio' where it is not associated with a value

a confusing crash deep inside the scheduler instead of the documented warn-and-default behavior its sibling classes give.

Fix

Normalize warmup_type in WarmupCosineLR.__init__ exactly as WarmupLR.__init__ already does (same warning text, same fallback to log). Behavior for valid log/linear values is unchanged.

Testing

Added test_warmup_cosine_lr_unknown_warmup_type_falls_back_to_log, which fails with the UnboundLocalError above on current master and passes with this change; it asserts an unknown warmup_type produces the same lr-ratio trajectory as an explicit log scheduler through warmup and into cosine decay.

pytest tests/unit/runtime/test_lr_schedulers.py -k "warmup_cosine or reject_invalid"
9 passed, 45 deselected

yapf/flake8 clean on both touched files. Follows up on the recent scheduler hardening in #8126 and #8142, which did not cover warmup_type.