Validate warmup_type in WarmupCosineLR like WarmupLR (#8151)
## 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`.
---------
Signed-off-by: Sohum Trivedi <trivsohum@gmail.com>
